sustainability

Review Environmental and Economic Sustainability of Swine Wastewater Treatments Using Ammonia Stripping and Anaerobic Digestion: A Short Review

Adele Folino 1, Demetrio Antonio Zema 1,* and Paolo S. Calabrò 2

1 Department Agraria, Mediterranea University of Reggio Calabria, Località Feo di Vito, I-89122 Reggio Calabria, Italy; [email protected] 2 Department Diceam, Mediterranea University of Reggio Calabria, Via Graziella, Località Feo di Vito, I-89124 Reggio Calabria, Italy; [email protected] * Correspondence: [email protected]



Received: 30 April 2020; Accepted: 16 June 2020; Published: 18 June 2020


Abstract: One of the most promising systems to treat swine wastewater is air stripping. This system simultaneously recovers nitrogen salts, to be used as fertiliser, and reduces the organic pollutant load in the effluents of swine breeding farms. Several reviews have discussed the air stripping as a treatment for many types of industrial wastewater or nitrogen-rich digestate (the liquid effluent derived from the anaerobic digestion plants) for the stripping/recovery of nutrients. However, reviews about the use of air stripping as treatment for raw or anaerobically digested swine wastewater are not available in literature. To fill this gap, this study: (i) Summarises the experiences of air stripping for recovery of ammonium salts from both raw and digested swine wastewater; and (ii) compares air stripping efficiency under different operational conditions. Moreover, combined systems including air stripping (such as struvite crystallisation, chemical precipitation, microwave radiation) have been compared. These comparisons have shown that air stripping of raw and digested swine wastewater fits well the concept of bio-refinery, because this system allows the sustainable management of the piggery effluent by extracting value-added compounds, by-products, and/or energy from wastewater. On the other hand, air stripping of raw and digested swine wastewater has not been extensively studied and more investigations should be carried out.

Keywords: air stripping; ammonia removal efficiency; ammonium sulphate; digestate; nitrogen recovery; process conditions; struvite

1. Introduction The management of swine wastewater (hereinafter indicated as “SW”) is an important problem for swine breeding farms, as the large production and polluting content lead to several environmental and economic constraints and a generally low sustainability. Due to the high contents of organic and nitrogen compounds, unproper disposal of SW causes pollution of surface and ground waters, unpleasant odour emission in atmosphere, and consumption of oxygen in soil and water bodies. Intensive depuration plants are the most common systems for SW treatment [1]. Other chemical (such as coagulation/flocculation [2–4]), physical [5], or biological (multi-stage or aerobic treatment [1,6]) systems [6–8] have been proposed to ensure an environmentally and economically sound management of SW (Table1). However, these treatments generally show low e fficiency, high costs [9], and process instability, mainly due to the high concentration of ammonia nitrogen that inhibits the microorganisms’ activity. When chemicals are added to the processes (e.g., coagulation/flocculation), the resulting sludge/concentrate cannot be directly spread in agricultural fields, as it may contain undesired

Sustainability 2020, 12, 4971; doi:10.3390/su12124971 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 4971 2 of 28 by-products of the chemical process or toxic compounds (e.g., heavy metals) [2,4]. Moreover, secondary pollution may occur [10] and other subsequent treatments may be necessary for the produced sludge/concentrate.

Table 1. Advantages and drawbacks of the main systems for the treatment of swine wastewater.

Treatment Type Advantages Limitations Water disinfection Microbes and solids can be removed by sedimentation Possible undesired by-products, such as or filtration after flocculation by the addition of trihalomethanes and chlorites in case of coagulants [2] chlorination [2] Chemical accumulation of nutrients Variability of process conditions depending on the Soluble nutrients bound to colloids precipitate as level of disinfection required [2] Coagulation – Flocculation – solids and separated by settling in clarifiers [4] Operating costs [11] Chemical Disinfection (possible) Removal of organic matter and other inorganic species, Sludge production and possible presence such as arsenic and fluoride [4] of heavy metals [11] Flexibility of the operational process. The use of Possible inhibitory effects in the simultaneous chemical precipitation in modified following biological treatment [12] activated sludge systems [11] Low nutrients availability and agronomic utilisation, Low capital cost for reducing phosphorous particularly with aluminium and iron coagulation [13] concentration in the effluent [11] Fertiliser production by bio-solid recovery [2] Salmonella typhimurium Pathogens reduction, such as , Energy costs of filtration membranes [4] Sedimentation E. coli Streptococcus faecalis , [2] Not suitable direct use of the concentrate due to the Physical Reduction of the organic load for the following accumulation of undesired contaminants [4] treatments [2] Pre-treatment needed to prevent membrane fouling [4] Selective separation of the constituents from High cost for membrane replacement due to fouling Membrane filtration waste streams based on the membrane used [4] Possible inhibition due to the presence of Odour control, nitrogen management, toxic compounds (such as resistant pathogens, heavy Biological Activated sludge process and biodegradation of organic waste [2] metals, other organic compounds) system Pathogens inactivation and/or removal [2] Process sensitivity to environmental conditions (pH, temperature, organic load)

Often, most of these treatments are not sustainable for the smallest swine-breeding farms working in rural contexts. For these reasons, much attention has been recently paid to extensive systems for SW depuration, such as the aerobic and/or anaerobic lagooning plants (e.g., [14–16]). These extensive systems allow a proper equalisation of the physico-chemical characteristics of wastewater and a reduction of its pollutant load, as for other agro-industrial effluents [17–19]. However, the physico-chemical and biological processes of lagooning plants require a long time (weeks or even months), because the environmental conditions cannot be properly set up. Another viable system for SW treatment is the anaerobic digestion, which allows an easier control of the main operational parameters of the depuration process. The anaerobic digestion systems are used for depurating several agro-industrial waste and wastewater, such as the olive mill wastewater or citrus peel waste and wastewater [20–22]. However, the methane production in the anaerobic digestion plants 1 treating SW is often reduced or even inhibited by the high ammonia content (often over 4.0 g L− ) that hampers the activity of the methanogenic bacteria [23,24]. As a consequence, the methane yield of 1 1 raw SW is about 50 mL gCOD− [23,25] or 100–200 mL gVS− [24,26]. As the SW has a high organic and nutrient loads, the treatment efficiency is not always high and the limits for discharging the treated effluents into water bodies are very strict in some countries (such as Italy), the depurated SW very often is spread on soil for agronomic purposes or sent to further treatments, such as lagooning and constructed wetlands. On the other hand, the high presence of ammonium and phosphorus compounds has suggested the recovery of natural fertilisers from SW using physico-chemical treatments. Examples of these recovered compounds are struvite (either in magnesium, MgNH4PO4, or potassium, KNH4PO4, form) [27,28] and ammonium sulphate [25]. Nitrogen and phosphorus are essential nutrients for plant growth and crop cultivation, but their production is often expensive. The removal of these compounds from SW not only allows their recovery, but also makes the following treatments easier, leading to a more efficient depuration in lagooning plants and to an increase of the methane yield in anaerobic digestion systems. Sustainability 2020, 12, 4971 3 of 28

The most common system for ammonia recovery is air stripping. This system consists of blowing air in wastewater, which is also chemically pre-treated by adding alkali—such as NaOH, Ca(OH)2, or CaO—in order to optimise its pH before aeration. The stripped ammonia is recovered by an adsorbing unit as ammonium sulphate, an inorganic salt that can be directly used as a fertiliser. Moreover, when air stripping is used as a pre-treatment for the SW, the risk of ammonia inhibition is lower; for the direct use of the digestate in agriculture, a simple solid–liquid separation without other chemical treatments is only required. An alternative plant scheme is the air stripping of digested wastewater. The effluents of anaerobic digestion, if influent SW is not pre-treated, usually show a high nitrogen content even after the anaerobic digestion. Moreover, the high water content and the possible presence of contaminants (such as heavy metals and organic matter) often makes the digestate not suitable for direct spreading on land [4]. In these cases, air stripping is also applied to reduce the ammonia nitrogen content, in order to allow the valorisation of the digestate as soil conditioner, thus increasing the sustainability of the whole management processes. The literature reports several reviews on AS of wastewater of different sources, such as municipal or industrial wastewater [4,27,29–32] as well as of urine [33] and manure [34], and about recovery of nutrients from digestate [35,36]. However, no reviews have been published on ammonia removal from raw or digested SW by air stripping. In order to fill this gap, this review analyses the AS techniques and their performances on both raw and digested SW. Based on the most important studies found in literature, the state of the art is outlined and the future research directions are suggested, to further improve the technical, economic, and environmental sustainability of AS of raw SW and digested SW.

2. Air Stripping Air stripping is one of the most common systems for removing volatile pollutants, especially ammonia, from aqueous solution [31], because it is a relatively simple and cost-effective process [32,37]. However, the industrial application of air stripping is still in its preliminary phase [31]. Thus, more research is needed to consolidate its practical use [4]. The removal efficiency of air stripping mainly depends on four parameters: (i) pH; (ii) temperature; (iii) air flow per unit volume of wastewater (hereinafter “air flow rate”, AFR); and (iv) characteristics of the raw wastewater [25,38,39]. The following sub-sections explain the effects of the operational parameters (pH, temperature, and AFR) on the removal efficiency of ammonia nitrogen and the main drawbacks in air stripping application; then, the processes of nutrient recovery and the alternative schemes of air stripping in literature are discussed.

2.1. Effects of the Operational Parameters on the Removal Efficiency of Ammonia Nitrogen and Main Drawbacks of Air Stripping Application Basically, air stripping consists of a mass transfer based on a desorption process, which transfers a gas from the liquid to a gas phase. For SW (the liquid phase), ammonia can be found as ammonium + ions (the ionic form, NH4 ) and ammonia gas (the molecular form, NH3)[32], which are in equilibrium according to the following reaction (Equation (1)) [32]:

+ NH + H O NH + OH− (1) 3 2 ⇔ 4 The distribution between molecular ammonia and ammonium ions, described by Equation (2) [39], depends on pH and temperature, as explained by Equation (3) [38]:

h +i NH3 + NH [ ] 4 NH3 = + (2) 1 + [H ]/Ka

8 3 5 2 pKa = 4 10− T + 9 10− T + 0.0356 T + 10.072 (3) · · SustainabilitySustainability 2020, 12, x FOR PEER REVIEW 4 of 27 SustainabilitySustainability 20202020,, 1212,, 2020xx FORFOR, 12 PEERPEER, x FOR REVIEWREVIEW PEER REVIEW 44 ofof 2727 4 of 27 SustainabilitySustainability 2020, 12, 2020x FOR, 12 PEER, x FOR REVIEW PEER REVIEW 4 of 27 4 of 27 air strippingair stripping application; application; then, the then, processes the processes of nutrient of nutrient recovery recovery and the and alternative the alternative schemes schemes of air of air airair strippingstrippingair stripping application;application; application; then,then, thethe then, processesprocesses the processes ofof nutrientnutrient of nutrient recoveryrecovery recovery andand thethe and alternativealternative the alternative schemesschemes schemes ofof airair of air strippingstripping in literature in literature are discussed. are discussed. strippingstrippingstripping inin literatureliterature in literature areare discussed.discussed. are discussed. 2.1. Effects2.1. ofEffects the Operational of the Operational Parameters Parameters on the Remova on the Removal Efficiencyl Efficiency of Ammoni of Ammonia Nitrogena Nitrogen and Main and Main 2.1.2.1. EffectsEffects2.1. ofofEffects thethe OperationalOperational of the Operational PaParametersrameters Parameters onon thethe RemovaRemova on the Removall EfficiencyEfficiencyl Efficiency ofof AmmoniAmmoni of Ammoniaa NitrogenNitrogena Nitrogen andand MainMain and Main DrawbacksDrawbacks of Air Stripping of Air Stripping Application Application DrawbacksDrawbacks of Air Stripping of Air Stripping Application Application Basically,Basically, air stripping air stripping consists consists of a mass of atransfer mass transfer based on based a desorption on a desorption process, process, which transferswhich transfers Basically,Basically, air stripping air stripping consists consists of a mass of atransfer mass transfer based on based a desorption on a desorption process, process, which transferswhich transfers a gas froma gas the from liquid the toliquid a gas to phase. a gas phase.For SW For (the SW liquid (the phase),liquid phase), ammonia ammonia can be foundcan be asfound ammonium as ammonium a gas froma gas the from liquid the to liquid a +gas to phase. a+ gas Forphase. SW For (the SW liquid (the phase), liquid phase), ammonia ammonia can be foundcan be asfound ammonium as ammonium ions (theions ionic (the form, ionic NH form,4 +) and NH ammonia4+) and ammonia gas (the gas molecular (the molecular form, NH form,3) [32], NH which3) [32], arewhich in equilibrium are in equilibrium ions (theions ionic (the form, ionic NH form,4 ) and NH ammonia4 ) and ammonia gas (the gas molecular (the molecular form, NH form,3) [32], NH which3) [32], arewhich in equilibrium are in equilibrium ions (theions ionic (the form, ionic NH form,4+) and NH ammonia4+) and ammonia gas (the gas molecular (the molecular form, NH form,3) [32], NH which3) [32], arewhich in equilibrium are in equilibrium accordingaccording to the following to the following reaction reaction (Equation (Equation (1)) [32]: (1)) [32]: accordingaccordingaccording toto thethe following followingto the following reactionreaction reaction (Equation(Equation (Equation (1))(1)) [32]:[32]: (1)) [32]: + − + − + NH⇔+ H O+ +⇔ NH− + + OH − NH3 +H2O ⇔3 +NH2 4+ ⇔+OH− 4+ + − (1) (1) NH3 + HNH2O⇔3 + NHH2O4 +⇔OHNH 4 + OH (1) (1) NH3 H2NHO 3 NHH2O4 OHNH4 OH (1) (1) The distributionThe distribution between between molecular molecular ammonia ammonia and ammonium and ammonium ions, described ions, described by Equation by Equation (2) (2) The distributionThe distribution between between molecular molecular ammonia ammonia and ammonium and ammonium ions, described ions, described by Equation by Equation (2) (2) [39],[39], dependsdepends[39], depends onon pHpH onandand pH temperature,temperature, and temperature, asas explainedexplained as explained byby EquationEquation by Equation (3)(3) [38]:[38]: (3) [38]: [39], depends[39], depends on pH andon pH temperature, and temperature, as explained as explained by Equation+ by Equation +(3) [38]: (3) [38]: [ + [ +] + +] [NH3 +NH[NH4+ ]3 +NH4+ ] Sustainability 2020, 12, 4971 []= [[]NH3 =NHNH4 ]3 NH4 4 of 28 []NH3 = []NHNH3++=NH[NH ++ NH ] (2) (2) []NH3 = []NH+[]33 += +4 []3 + 4 (2) (2) NH3 1NH+[]H3 + /1K+a []H + / Ka (2) (2) 1+[]H /1K+a[]H / Ka 1 H /1Ka H / Ka −8 3 −8 −35 2 −5 2 pK = 4 ⋅pK10 −8T= 34+⋅109 ⋅−108T−35T+ 29+⋅100.0356−5T 2 +T0+.035610.072T + 10.072 a = ⋅pK−a8 =34+⋅10⋅−8 T−53 +29+⋅10−5 T 2 + 0.+0356 T + 10.072 (3) (3) pK a = 4⋅ 10 a T= +⋅ 9⋅ 10 T+ +⋅ 0.0356+T + 10.072 + (3) (3) pK a 4 pK10 a T 4 109 10T T 9 100.0356T T0.035610.0721 T 10.072 + (3) where [NH3] is the molecular ammonia concentration [mol L− ], [NH3 + NH4 ] is the total ammonia(3) −1 −1 + + where where[NH3] is[NH the3 ]molecular is the molecular ammonia ammonia concentration concentration [mol L [mol−1], [NH L−13], + [NH NH34 ++] isNH the4+ ]total is the ammonia total ammonia where where[NH3] is[NH the3] molecular is the molecular ammonia 1ammonia concentration+ concentration [mol L [mol], [NH L 3], + [NH NH34 +] isNH the4 ] total is the ammonia total ammonia1 3 3 −1 −13 43+ 4+ nitrogenwhere (TAN) [NHwhere concentration] is[NH the ]molecular is the molecular [mol ammonia L− ammonia−1], concentration [H+ −1] isconcentration the+ hydrogen[mol L [mol], [NH L ion], + [NH concentrationNH ]+ isNH the ]total is− 1the [molammonia total−1 Lammonia− ], Ka is the nitrogennitrogen (TAN) (TAN)concentration concentration [mol L [mol−1], [H L+]− 1is], [Hthe+] hydrogenis the hydrogen ion concentration ion concentration [mol L [mol−1], Ka L is−1], the Ka is the nitrogennitrogen (TAN) (TAN)concentration concentration [mol L −[mol1],], [H[H L+]]− 1 is],is [Hthethe+ ] hydrogen hydrogenis the hydrogen ionion concentrationconcentration ion concentration [mol[mol LL −[mol1],], KaKa L is−is1], thethe Ka is the acid ionizationacid ionizationacid constant, ionization constant, and constant, and T is T theis and the temperature T temperature is the temperature [°C] [◦C] [40]; [[°C]40 pK]; [40];a pKis equal pKa isa is equalto equal −log 10to (K −alog) log[dimensionless].10(K10a)(K [dimensionless].a) [dimensionless]. acid ionizationacid ionization constant, constant, and T is and the T temperature is the temperature [°C] [40]; [°C] pK [40];a is equal pKa is to equal −log 10to(K −loga) [dimensionless].10(Ka) [dimensionless]. acid ionizationacid ionization constant, constant, and T is and the T temperature is the temperature [°C] [40]; [°C] pK [40];a is equal pKa is to equal −log 10to(K −−alog) [dimensionless].10(Ka) [dimensionless]. If the pHIf the is acidic,pHIf the is acidic,pH neutral, is acidic,neutral, or neutral, or slightly slightly or slightly alkalinealkaline alkaline (< (8.0–8.5),<8.0–8.5), (< 8.0–8.5), the equilibrium the the equilibrium equilibrium reaction reaction reactionshown shownin the shown in the in the IfIf thethe pHpHIf the isis acidic,acidic,pH is acidic,neutral,neutral, neutral, oror slightlyslightly or slightly alkalinealkaline alkaline (<(< 8.0–8.5),8.0–8.5), (< 8.0–8.5), thethe equilibriumequilibrium the equilibrium reactionreaction reaction shownshown showninin thethe in the EquationEquation (1) is displaced(1) is displaced to the toright, the while,right, while,if pH increasesif pH increases (> 8.5), (> the 8.5), formation the formation of molecular of molecular EquationEquation (1) isEquation displaced (1) is displaced(1) tois thedisplaced to right, the toright, while, the while,right, if pH while,if pH increases increasesif pH increases (> (>8.5), 8.5), the (>the 8.5), formation formation the formation of molecular of molecular ammonia ammoniaammonia is enhanced is enhanced (Table 2)(T able[31,40–42]. 2) [31,40–42]. is enhancedammoniaammonia (Tableammonia isis enhancedenhanced2)[31 is enhanced,40 (T(T–42ableable]. 2)(T2) able[31,40–42].[31,40–42]. 2) [31,40–42].

Table 2.Table Effects 2. ofEffects variations of variations of parame of tersparameters onon airairters strippingstripping on air stripping system.system. system. TableTable 2. E 2.ffTable ectsEffects of2. of Effects variations variations of variations of of parame parameters of tersparame on airters on stripping on air air stripping stripping system. system. system. Effect Effect ParameterParameter Variation Variation Effect Effect ParameterParameterParameter Variation Variation Variation on Air Eff onStrippingect Air on AirStripping Stripping onon AirAir onStrippingStripping Air Stripping pH pH pH pH pH Temperature TemperatureTemperatureTemperature TemperatureTemperature Air flowAir rate flow rate Air flowAir flow rateAir rate flow rate A schemeA scheme of the airof thestripping air stripping process process is shown is shownin Figure in Figure1. As the 1. Asaim the of aimthe treatmentof the treatment is the is the A schemeA scheme of the airof thestripping air stripping process process is shown is shownin FigureFigure in Figure1.1. AsAs thethe 1. Asaimaim the ofof aimthethe treatmenttreatmentof the treatment isis thethe is the removalremovalremoval ofof ammoniaammonia of ammonia nitrogennitrogen nitrogen compcompounds comp (bothounds the (both ionic the and ionic molecular and molecular forms) forms)from the from SW, the the SW, pH the pH removalremoval of ammonia of ammonia nitrogen nitrogen compounds comp (bothounds the (both ionic the and ionic molecular and molecular forms) forms)from the from SW,+ the the SW, pH+ the pH increaseincrease over a overcertain a certainthreshold threshold allows allowsthe conversion the conversion of almost of almostall the allcontent the content of NH 4of+ into NH 4the+ into the increaseincrease over a overcertain a certainthreshold threshold allows allowsthe conversion the conversion of almost of almostall the allcontent the content of NH 4of into NH 4the into the A schemeincreaseincrease ofover the a overcertain air strippinga certainthreshold threshold processallows allowsthe is conversion shown the conversion in of Figurealmost of almost1all. Asthe allcontent the the aim content of ofNH the 4of+ into NH treatment the4+ into the is the gaseousgaseous form, NH form,3. Hence, NH3. Hence, the first the step first of stepthe processof the process genera llygenera consistslly consists of the additionof the addition of an alkali of an alkali gaseousgaseous form, NH form,3. Hence, NH3. Hence, the first the step first of stepthe processof the process genera llygenera consistslly consists of the additionof the addition of an alkali of an alkali gaseousgaseous form, NH form,3. Hence, NH3. Hence,the first the step first of stepthe process of the process genera llygenera consistslly consists of the addition of the addition of an alkali of an alkali removalsolution of ammoniasolution to the raw to nitrogen the SW, raw in SW,order compounds in to order increase to increase (bothits pH. theitsThen, pH ionic .in Then, the and second in the molecular second unit, the unit, mass forms) the transfer mass from transfer of NH the3 of SW, NH the3 pH solutionsolution to the raw to the SW, raw in SW,order in to order increase to increase its pH. itsThen, pH .in Then, the second in the second unit, the unit, mass the transfer mass transfer of NH3 of NH3 solutionsolution to the raw to the SW, raw in orderSW, in to order increase to increase its pH. itsThen, pH .in Then, the second in the secondunit, the unit, mass the transfer mass transfer of NH3 of +NH3 increasefrom over SWfrom a certainis enhancedSW is enhanced threshold by the aerationby allowsthe aeration that the alters that conversion the alters air–liquid the air–liquid of boundary almost boundary [31]. all theTemperature [31]. content Temperature plays of NH an plays 4 aninto the fromfrom SWSWfrom isis enhanced enhancedSW is enhanced byby thethe aerationbyaeration the aeration thatthat altersalters that thethe alters air–liquidair–liquid the air–liquid boundaryboundary boundary [31].[31]. TemperatureTemperature [31]. Temperature playsplays anan plays an importantimportant effect oneffect the onremoval the removal of molecular of molecular ammonia am moniafrom wastewater, from wastewater, since thesince solubility the solubility of of gaseousimportant form,importantimportant NH effecteffect3. Hence, ononeffect thethe on theremovalremoval the first removal ofof step molecularmolecular ofof themolecular amam processmonia am moniafrom generally wastewater, from consistswastewater, since of the since the solubility additionthe solubility of of anof alkali ammoniaammonia in water, in accordingwater, according to Henry’s to Henry’s law, is govelaw, isrned gove byrned the temperatureby the temperature as well asas wellby solute as by andsolute and solutionammoniaammonia to theammonia raw inin water,water, SW, in inaccordingwater,according order according to to Henry’sHenry’s increase to Henry’s law,law, its isis pH.govelaw,gove rnedisrned Then, gove bybyrned inthethe the temperaturetemperatureby the second temperature asas unit, wellwell theas asas well byby mass solutesolute as by transfer and andsolute and of NH solventsolvent amounts amounts [27,32,40,43]. [27,32,40,43]. Heating Heating wastewater wastewater enhances enhances the diffusion the diffusion of the ammonia of the ammonia molecules molecules 3 solventsolventsolvent amountsamounts amounts [27,32,40,43].[27,32,40,43]. [27,32,40,43]. HeatingHeating Heating wastewaterwastewater wastewater enhancesenhances enhances thethe diffusiondiffusion the diffusion ofof thethe ammoniaammonia of the ammonia moleculesmolecules molecules from SWto isthe enhanced surfaceto the surface of the by solution,of the the aerationsolution, and then and that to then the alters atmoto thesphere the atmo air–liquid sphere[31]. In [31]. the boundarycaseIn the of caseair stripping, of [ 31air]. stripping, Temperature the NH the3 NH3 plays to the surfaceto the surface of the solution,of the solution, and then and to then the atmoto thesphere atmo sphere[31]. In [31]. the caseIn the of case air stripping, of air stripping, the NH the3 NH3 to the surfaceto the surface of the solution,of the solution, and then and to then the atmoto thesphere atmo sphere[31]. In [31]. the caseIn the of caseair stripping, of air stripping, the NH the3 NH3 an importanttransfertransfer e fromffect wastewateron from the wastewater removal to the air toof theis facilitated molecular air is facilitated by the ammonia airby thebubbles. airfrom bubbles. Besides wastewater, Besides the pH the and pH temperature, since and temperature, the solubility of transfertransfertransfer fromfrom wastewaterwastewater from wastewater toto thethe air airto istheis facilitatedfacilitated air is facilitated byby thethe airbyair bubbles.thebubbles. air bubbles. BesidesBesides Besides thethe pHpH the andand pH temperature,temperature, and temperature, the AFRthe is AFRanother is another important important parameter parameter that influe thatnces influe thences removal the removal rates (mass rates transfer) (mass transfer) of ammonia of ammonia ammoniathethe in AFRAFR water,the isis AFRanotheranother according is another importantimportant important to parameterHenry’sparameter parameter law,thatthat influeinflue is that governednces influe thences removal by the the removal rates temperature (mass rates transfer) (mass as transfer) of well ammonia asof ammonia by solute and from SW,from as SW, it establishes as it establishes a high agradient high gradient of ammonia of ammonia concentration concentration between between the liquid the andliquid air and air solvent amountsfromfrom SW,SW,from [asas27 SW, itit,32 establishesestablishes ,as40 it,43 establishes]. Heatingaa highhigh agradientgradient high wastewater gradient ofof ammoniaammonia of enhances ammonia concentrationconcentration theconcentration diff betweenusionbetween between of thethe the liquidliquid ammonia the andliquidand airair moleculesand air to phases phases[32,38]. [32,38]. This process This process is known is known as ’diffusion’: as ’diffusion’: The molecules The molecules (free ammonia (free ammonia in this incase) this case) phases phases[32,38]. [32,38]. This process This process is known is known as ’diffusion’: as ’diffusion’: The molecules The molecules (free ammonia (free ammonia in this incase) this case) the surfacespontaneously of thespontaneously solution, mix an, andmix moving an, then moving from to the regions from atmosphere regions of rela tivelyof [rela31 high].tively In concentration the high case concentration of airinto stripping, regions into regions of lower the of NH lower3 transfer spontaneouslyspontaneouslyspontaneously mixmix an,an, mix movingmoving an, moving fromfrom regionsregions from regions ofof relarela tivelytivelyof rela highhightively concentrationconcentration high concentration intointo regionsregions into regions ofof lowerlower of lower from wastewaterconcentrationconcentration to the[44], airblowing [44], is facilitated blowing air in the air SW. byin the theTherefore, SW. air Therefore, bubbles. the transfer the Besides transfer of ammonia theof ammonia pH from and the from temperature, wastewater the wastewater is the is AFR is concentrationconcentrationconcentration [44],[44], blowingblowing [44], blowing airair inin thethe air SW.SW. in the Therefore,Therefore, SW. Therefore, thethe transfertransfer the transfer ofof ammoniaammonia of ammonia fromfrom thethe from wastewaterwastewater the wastewater isis is stronglystrongly improved improved by the byair theflow. air The flow. water The vapourwater vapour stripped stripped by the byair theflow air is flowgenerally is generally collected collected another importantstronglystronglystrongly improvedimproved parameter improved byby thethe airbyair that theflow.flow. influences air TheThe flow. waterwater The the vapourvapourwater removal vapour strippedstripped stripped rates byby thethe (mass airbyair theflowflow transfer) air isis flow generallygenerally is ofgenerally ammoniacollectedcollected collected from SW, into a thirdinto aunit third (condenser), unit (condenser), while thewhile gaseous the gaseous ammonia ammonia flows into flows the into last the unit last (trap), unit containing(trap), containing as it establishesintointo aa thirdthirdinto a ahigh unitunit third (condenser),(condenser), gradient unit (condenser), of whilewhile ammonia the thewhile gaseousgaseous the concentration gaseous ammoniaammonia ammonia flowsflows between into intoflows thethe into lastlast the the unitunit liquid last (trap),(trap), unit and containing(trap),containing air containing phases [ 32,38]. an acidican solutionacidic solution (generally (generally H2SO4), H where2SO4), thewhere ammonia the ammonia nitrogen nitrog is recovereden is recovered as salt. as salt. an acidican solutionacidic solution (generally (generally H2SO4), H where2SO4), thewhere ammonia the ammonia nitrogen nitrog is recovereden is recovered as salt. as salt. This processan acidic isan known solution acidic solution as(generally ’diff (generallyusion’: H2SO4 The), H where2SO molecules4), thewhere ammonia the (free ammonia nitrog ammoniaen nitrog is recovereden in is thisrecovered as case) salt. as spontaneously salt. mix an, moving from regions of relatively high concentration into regions of lower concentration [44], blowing air in the SW. Therefore, the transfer of ammonia from the wastewater is strongly improved by the air flow. The water vapour stripped by the air flow is generally collected into a third unit (condenser), while the gaseous ammonia flows into the last unit (trap), containing an acidic solution (generally

H2SO4), whereSustainability the 2020 ammonia, 12, x FOR PEER nitrogen REVIEW is recovered as salt. 5 of 27

Figure 1. FigureScheme 1. Scheme of the of airthe air stripping stripping process and and recovery recovery of ammonium of ammonium sulphate assulphate by-product as(SW by-product = swine wastewater; DSW = digested swine wastewater). (SW = swine wastewater; DSW = digested swine wastewater). Typical literature values of the parameters influencing air stripping are pH between 10.0 and 11.0, temperature up to 80 °C, and air flow rate from 0.5 to 10 L L−1 min−1.

2.2. Processes of Nutrient Recovery and Alternative Schemes of Air Stripping Although air stripping theoretically is an efficient method to remove ammonia from wastewater, this process shows some drawbacks, such as the fouling of the ammonia stripping tower, production of sludge, and release of ammonia gas [32]. Fouling of stripping tower is usually due to the to the formation of salts (e.g., calcium carbonate) that scale the surface of the packing materials [32], but also to the presence of suspended solids. The preliminary phase of solid–liquid separation is extremely important to avoid this problem, as most of the suspended solids can be retained in the solid fraction after separation [36]. Moreover, the high cost of the air stripping process may limit its application in the future [4]. Chemicals, such as alkalis for raising up the pH of the treated wastewater over 9.5, [4] and acids for lowering the pH of the post-stripping sludge, are needed during air stripping. This need is another issue of the process [27], since alkalis (such as lime, NaOH, or Ca(OH)2) are quite cheap, but high doses can increase the overall cost of air stripping [45]. For this reason, the pH optimisation is advised to balance the efficiency and cost of the process [32]. On the other hand, the effect of the different alkali type on the ammonia removal efficiency is not significant [46]. The pH adjustment may be necessary to meet the required limits for the disposal of the treated wastewater in sewage or water bodies, thus increasing the treatment costs [32]. The latter can be increased, when pre-heating (in the case of high-temperature process, sometime close to 80 °C [47]) and/or high aeration rates (up to 10 Lair Lwastewater−1 min−1) are required, to fasten the process [4]. Besides the process cost, the emission of the stripped ammonia to the atmosphere is a serious environmental issue [32]. For this reason, the ammonia adsorption after air stripping allows its conversion in the form of salt and this prevents the direct release of the ammonia into the environment [32]. This is a sustainable option for the valorisation of nitrogen in wastewater [48], since, as mentioned above, the recovered ammonium sulphate is marketable as fertiliser in agriculture. This option is one of the main advantages of air stripping, as detailed below. However, the ammonia concentration in the wastewater is another important factor affecting air stripping efficiency. In order to ensure a high concentration gradient for ammonia diffusion, aeration should be increased with increasingly high ammonia concentration [31] (thus increasing the energy cost). Air stripping of wastewater with low ammonia concentration (less than 2 g L−1) is not economically Sustainability 2020, 12, 4971 5 of 28

Typical literature values of the parameters influencing air stripping are pH between 10.0 and 11.0, 1 1 temperature up to 80 ◦C, and air flow rate from 0.5 to 10 L L− min− .

2.2. Processes of Nutrient Recovery and Alternative Schemes of Air Stripping Although air stripping theoretically is an efficient method to remove ammonia from wastewater, this process shows some drawbacks, such as the fouling of the ammonia stripping tower, production of sludge, and release of ammonia gas [32]. Fouling of stripping tower is usually due to the to the formation of salts (e.g., calcium carbonate) that scale the surface of the packing materials [32], but also to the presence of suspended solids. The preliminary phase of solid–liquid separation is extremely important to avoid this problem, as most of the suspended solids can be retained in the solid fraction after separation [36]. Moreover, the high cost of the air stripping process may limit its application in the future [4]. Chemicals, such as alkalis for raising up the pH of the treated wastewater over 9.5, [4] and acids for lowering the pH of the post-stripping sludge, are needed during air stripping. This need is another issue of the process [27], since alkalis (such as lime, NaOH, or Ca(OH)2) are quite cheap, but high doses can increase the overall cost of air stripping [45]. For this reason, the pH optimisation is advised to balance the efficiency and cost of the process [32]. On the other hand, the effect of the different alkali type on the ammonia removal efficiency is not significant [46]. The pH adjustment may be necessary to meet the required limits for the disposal of the treated wastewater in sewage or water bodies, thus increasing the treatment costs [32]. The latter can be increased, when pre-heating (in the case of high-temperature process, sometime close to 80 ◦C[47]) and/or high aeration rates (up to 10 Lair 1 1 Lwastewater− min− ) are required, to fasten the process [4]. Besides the process cost, the emission of the stripped ammonia to the atmosphere is a serious environmental issue [32]. For this reason, the ammonia adsorption after air stripping allows its conversion in the form of salt and this prevents the direct release of the ammonia into the environment [32]. This is a sustainable option for the valorisation of nitrogen in wastewater [48], since, as mentioned above, the recovered ammonium sulphate is marketable as fertiliser in agriculture. This option is one of the main advantages of air stripping, as detailed below. However, the ammonia concentration in the wastewater is another important factor affecting air stripping efficiency. In order to ensure a high concentration gradient for ammonia diffusion, aeration should be increased with increasingly high ammonia concentration [31] (thus increasing the energy cost). Air stripping of 1 wastewater with low ammonia concentration (less than 2 g L− ) is not economically sustainable [4,29]. The production of the fertiliser obtained by air stripping (ammonium sulphate) strictly depends on the concentration of ammonia in wastewater (the recovery of the ammonia salt is explained in the following Section 2.2.1). Therefore, a limited production of marketable fertiliser makes the process not economically convenient.

2.2.1. Ammonium Sulphate Recovery by Air Stripping Ammonium sulphate can be simply recovered by an adsorbing unit (acidic trap) immediately after the gas stripping phase [29,47,49,50] (Figure1). The amount of ammonium sulphate recovered can be determined by the stoichiometric Equation (4):

2NH + H SO (NH ) SO (4) 3 2 4 → 4 2 4 In this equation, it is hypothesized that the ammonium ions in the wastewater sample are totally 0 converted to ammonia gas. The Gibbs’ free energy of formation (∆fG ) under standard conditions (pH = 7 and 25 C) of the ammonium sulphate is 596.52 kJ mol 1. Since the Gibbs’ free energy is ◦ − − negative, the product formation in the Equation (4) is spontaneous. The production of the ammonium sulphate not only allows the recovery of nitrogen, but also makes sulphur available as macronutrient. The products recovered by the acidic trap generally include ammonium sulphate, concentrated Sustainability 2020, 12, 4971 6 of 28 ammonia solution, or other ammonia salts, such as ammonium nitrate [4]. Ammonium nitrate (NH4NO3) results from the reaction between NH3 and nitric acid and HNO3,[51]), according to the following reaction (Equation (5)) [52]:

NH + HNO NH NO (5) 3 3 → 4 3 As for the ammonium sulphate, this is a spontaneous reaction (∆ G0 = 53.88 kJ mol 1). f − − High quality and/or purity of these products should be ensured for reaching the market standards [36]. In the case of nitrogen recovery by air stripping, the use of sulphuric acid of high quality is sufficient to recover high purity by-products [36]. The amount of sulphuric acid required affects the S concentration in the salt; salt pH can largely vary from about 3.0 to 7.5 [51]. In general, low values of pH may lead to corrosion of machinery, leaf burning, and soil acidification in case of long-term application [53], while high pH may cause NH3 volatilisation during the fertilisation [51]. The acidic trap usually consists of a packed tower, in which the acid is sprayed by nozzles over the packing material and ammonia-enriched air is blown into the tower in counter current. At the real scale, the result is a concentrated solution of ammonium sulphate (with salt concentration of about 1 30%) marketed as liquid fertiliser at 90 to 120 € ton− [36,53,54]. This price is lower compared to the cost 1 of crystallised ammonium sulphate (about 140 € ton− [55,56]), since the market price mainly depends on the particle size of the salt [36,57]. The difficulty to obtain large-sized ammonium sulphate is mainly due to the construction, operational, and maintenance costs of the crystalliser vessel. Producing small-sized (about 0.4–1.0 mm) crystals of ammonium sulphate is more sustainable, compared to higher sizes (up to 2.6 mm), in terms of initial capital cost, energy requirement, and plant management [57]. In order to evaluate the purity of the produced ammonium sulphate, some physico-chemical analyses are required. In particular, the melting point of the salt should be equal to 240 ◦C and the nitrogen and sulphur content should be equal to 20% and 24%, respectively [58]. Nitrogen is determined by Kjeldahl method, while sulphur by gravimetric determination. However, ammonium sulphate produced from wastewater may show organic contamination (impurities), which affects the quality and the value as fertiliser [48]. In this case, biofilters can be used as a supplementary treatment to control volatile organic matter presence [38]. On the other hand, impurities (such as aluminium sulphate) were found to increase the crystallisation kinetics and size of the produced ammonium sulphate [59]. This was probably due to the different adsorption equilibria of the impurities on the crystal. Hence, the control of impurity concentrations may lead to the optimisation of crystal size and quality [59]. Also the concentrated ammonia solution (under non-crystallised form) is marketed, because the technical and economic sustainability of ammonium sulphate production under granular form has not been extensively investigated [36]. On the other hand, the higher nitrogen concentration in the salt reduces the transport costs of these products compared to the animal manure [51]. The ammonium sulphate can replace the synthetic fertilisers, since it is rich in nitrogen and sulphur, which are crop macronutrients [36,53]; this compound is particularly recommended for soils with alkaline or neutral reaction [32,35]. However, nitrogen recovery by air stripping or other treatments is more expensive than the production of synthetic ammonia fertiliser. The latter product is obtained by the well-consolidated Haber–Bosch process [4]. The energy requirement of this process 1 is around 10 kWh kg− [50,60], which is practically equal to the value of the air stripping process 1 (9 kWh kg− of nitrogen) [29,49,60]. The ammonium sulphate can be recovered from raw wastewater and digestate from anaerobic + 3 + digestion, thanks to the release of nutrients in soluble form (e.g., N–NH4 , P–PO4 −, and K–K ) during the anaerobic biodegradation [4]. However, the digestate from animal manure can be subjected to the strict regulation for fertilisers issued by some countries, such as the European Union. Under this point of view, processing of digestate for nutrient recovery can comply with these regulation constraints, and this turns the SW disposal problem into an economic opportunity [36]. It must be pointed out that the utilisation of ammonium sulphate from animal manure is also often officially regulated (e.g., 170 kg 1 1 of nitrogen ha− yr− ) according to the “Nitrate Directive” for the EU’s countries (91/676/CEE) [51,61]. Sustainability 2020, 12, 4971 7 of 28

In the United States, the standards for agricultural waste management are reported in the National Handbook of Conservation Practice Standards (CNMPs) [62], drafted in accordance with the conservation planning policies of the US Natural Resources Conservation Service (NRCS) and the guidance of the Environmental Protection Agency (EPA). Moreover, the following important standards Conservation Practice Standards (CPS), that define the minimum level of quality by which these practices are planned, operated, and maintained, are shown in the Agricultural Waste Management Field Handbook [63]: Nutrient Management (Code 590); Amendments for the Treatment of Agricultural Waste (Code 591); Waste Treatment (Code 629); Waste Utilization (Code 633). With regards to Latin America, every country establishes its own regulations. For instance, prevention for water pollution in Bolivia is regulated by the Law 1333 [64], while in Argentina, the protection of water quality and control of pollution are regulated by the Decree 674/89 and 776/92. In general, most of the countries of Latin America adopt regulations based on the EPA’s guidelines [65]. In Asian countries, e.g., China, the development of intensive farming required supply of nutrients in livestock manure; the uncontrolled discharge of manure waste, including SW, into the environment was facilitated by the diversity of regulations adopted by the different cities [66]. The allowed ammonium concentration discharge is regulated by the Discharge Standard of Water Pollutant for Ammonia Industry (GB 13458–2001) [67,68]. However, economic aspects may limit the diffusion of these technologies [68]. The agronomic effects of the ammonium sulphate produced by air stripping and used as soil fertiliser must be compared to the same product synthesised by the industry. In this regard, the literature complains unsatisfactory results, as more attention has been paid to the recovery technologies [51] than to the quality and agronomic effects of recovered nutrients. Szymanska et al. [69] carried out an experiment in pots on maize and grass in silty loam and loamy sand soils. Compared to soil without fertilisation, these authors found crop yields under the recovered fertiliser (88% and 73% for maize and grass, respectively) that were similar as the yields under the commercial products (125% and 94%, respectively) regardless of the type of soil. Similar results were obtained by Sigurnjak et al. [51] in terms of fertilising effects of ammonium sulphate compared to the synthetic nitrogen fertilisers. Moreover, this study showed that the nitrogen in the recovered ammonium sulphate is entirely under the mineral + form (100% as NH4 -N), such as the synthetic nitrogen fertiliser. The absence of phosphorous in ammonium sulphate can be considered a positive aspect, because the nitrogen requirements of crops can be satisfied without exceeding the phosphorous application rates [51].The results obtained in the literature suggest that the ammonium sulphate recovered in bio-refinery can replace the industrial ammonium sulphate used as fertiliser in crop cultivation with evident economic and environmental benefits [51,69].

2.2.2. Air Stripping Applications in Literature Some applications of air stripping at full scale have been found in the literature (Table3). The systems shown in Table3 were applied to di fferent waste streams (such as digestate, industrial wastewater, manure) and with different schemes for ammonia nitrogen recovery. Air stripping columns and packed tower are the most common configurations used for the treatment of air streams (such as acidic gases, alcohols, or solvents). In EU countries, companies propose their own patented systems that are more specifically addressed to the treatment of digestate from anaerobic digestion plant and to the recovery of ammonia as fertiliser. For instance, the AMFER® system has been used as pre-treatment of N-rich manure before the anaerobic digestion, thus increasing the capacity of feeding co-substrates to digesters; RECOV’AMMONIA™ ensures removal efficiencies over 90%. Nitrogen removal systems in Asian countries (e.g., China) are rarely implemented, in spite of the continuous growth of animal farms (especially chicken) and anaerobic digestion plants for treating manure [70]. Sustainability 2020, 12, 4971 8 of 28

Table 3. Examples of full-scale applications or constructors of air stripping systems in EU, USA, and Asia.

Constructor/ Country Type of Wastewater Systems/Applications Plant Site Closed loop system The discharged air is treated with an acid wash to form a salt from the ammonia and the air is 3 1 USA Branch Environmental Corporation [71] Industrial wastewater reused in the stripper.Capacities up to 681 m h− Air Stripping Columns Used for relatively highly volatile organic compounds (VOC) in a liquid stream. VOCs evaporate by flowing and mixing the water using an air flow. Packed Towers Used for the absorption of a variety of gases and solvents, such as acidic gas, alcohol and ammonia. AMFER® Digestate Nitrogen recovery from waste and wastewater flows with high levels of ammonium. No need of Colsen (The Netherlands) [72] Manure de-watering or of pre-treatment of the waste flow. It consists of just one process step. CO2 and EU- Highly polluted wastewater NH3 are successively removed from the substrate in a stripping column. The stripping air is passed through a gas scrubber, where ammonium sulphate or ammonium nitrate is produced. ANAStrip® plant Removal equal to 65% of the total nitrogen in the mineral fertilisers; recovery of calcium GNS (Röblingen am See, DE) [73] Digeste carbonate; recovery of heat from the process. 3 1 + 1 Technical operational process: Digestate input [m h− ]: 5.5–12.6; NH4 -N [g L− ]: 3–6; 1 1 Ammonium sulphate output [t d− ]: 13–27; Calcium carbonate output [t d− ]: 4–8 RECOV’AMMONIA™ Digestate + 1 CMI Europe Environnement (France) [74] Removal efficiency: > 92%; Technical operational process: Pollutants: NH4 at 2,4 g L− for a 102 Polluted liquid waste 1 3 1 3 1 kg h− flow; Liquid flow: 42 m h− at 60◦C and pH = 9; Process gas flow: 80,000 m h− Asia DQY biogas project (Beijing, China) Raw manure Recovery of ammonia before anaerobic digestion plant treating 100% chicken manure [75] Sustainability 2020, 12, 4971 9 of 28

Full-scale stripping processes (extensively described in [36]) have been also developed without internal packing. Some of these technologies can even work with relatively high suspended solids or without chemicals addition, giving a higher sustainability under both economic and environmental points of view [36]. The removal efficiency of these systems generally does not exceed 90% (although they are theoretically designed for a total removal), to reduce the operating costs [36]. On the other hand, a value of 90% of ammonia reduction in the effluent is generally considered acceptable. Several experiences with advanced air stripping have been carried out at the laboratory scale, employing schemes and technologies that could be used in future full-scale applications. The volume of the stripping unit at lab-scale ranges from 0.75 to 5 L [38,76,77]. Complex air stripping schemes have been developed, in order to overcome the operational problems previously mentioned and increase the ammonia removal efficiency. For example, the use of vacuum can facilitate the gas stripping [78–80], as vacuum lowers the boiling point of the liquid. Different schemes have been developed to overcome the fouling problems of the packed towers for large-scale wastewater treatment [27]. A water-sparged aerocyclone was proposed by Quan et al. [43]; the aerocyclone was more efficient compared to the traditional system in terms of energy saving and removal efficiency. A jet loop reactor configuration was used by Degermenciˆ et al. [81] to minimise the aeration energy requirement, since the mass transfer capability of this reactor was higher than other reactor types, even if less air was supplied, due to the high internal mixing and larger contact area. Yuan et al. [82] used a continuous-flow rotating packed bed at lab- and pilot-scale at room temperature; this system was able to maximise the ammonia removal efficiency and reduce the treatment time. Biological treatments are often used in combination with air stripping (Figure2). An aerobic treatment was used by Alitalo et al. [83] to increase the pH of pig slurry without chemical addition, and the combined process removed over 30% of ammonia. The temperature was in the mesophilic range (35–37 ◦C) and chemicals (MgO and Ca(OH)2) were added in the subsequent stripping cycles, raising the ammonia removal efficiency up to 86%. Aerobic species generally able to treat swine wastewater are Bacillus amyloliquefaciens, Bacillus cereus, Pseudomonas sp., Paenibacillus sp., and uncultured Actinobacteria bacterium [8]. In a study of Yang et al. [84], the air stripping was used to increase acidogenesis as first stageSustainability for the 2020 subsequent, 12, x FOR PEER methanogenesis REVIEW in a two-stage anaerobic digestion system. In fact, acidogens10 of 27 play the primary role in producing major substrates for methanogens [84].

Air stripping as pre-treatment Air stripping as post-treatment

(a) (b) (c)

Figure 2. Schemes and limitations of air stripping ( a) coupled with microalgaemicroalgae cultivationcultivation system (adapted from Cao et al. [[10]),10]), ((b)) asas pre-treatmentpre-treatment andand ((c)) asas post-treatmentpost-treatment ofof anaerobicanaerobic digestiondigestion (SW = swineswine wastewater; wastewater; AD AD = anaerobicanaerobic digestion). digestion).

Air stripping has been well integrated in bio-energy production systems, as this process can be applied on digestate after biogas production (thus for biofuel and/or heat production) from organic waste or as treatment prior to the anaerobic digestion (Figure 2). As a matter of fact, air stripping enhances methane yield, apart from being cost-effective for ammonia removal [32]. When air stripping is used as pre-treatment of anaerobic digestion, the main purpose is the removal of ammonia compounds that can inhibit the methanogenic activity. In this case, the dose and type of alkalis for initial pH adjustment of SW should be appropriately evaluated, in order to avoid possible inhibition of the anaerobic digestion due to cationic toxicity [25] or to an excessive nitrogen removal. The anaerobic digestion of raw SW, in addition to the difficulties mentioned above, may lead to the production of a digestate containing organic residues and ammonia compounds. Hence, a further biological (aerated) process is generally required to allow the use of the digestate as soil conditioner. To overcome this issue, air stripping can also be applied as post-treatment of anaerobic digestion to reduce the nitrogen and organic concentrations in the digestate. The stripped ammonia from both configurations (that is, from raw and digested SW) can be recovered as ammonium sulphate. Moreover, anaerobic digestion is another way to recover carbon (generally expressed in terms of chemical oxygen demand, COD) from swine wastewater, since the biodegradable carbon can be decomposed by microorganisms [85]. However, as previously mentioned, the theoretical methane yield (about 380 mL gCOD−1, [86] corresponding to a complete conversion of COD), cannot be achieved due to the presence of inhibiting ammonia compounds. When air stripping is coupled with anaerobic digestion (Figure 2, b,c), relatively high temperatures (up to 60 °C) can be more easily achieved using the excess heat of the combined heat and power plant [87] or recovering heat from flue gas in conventional schemes of electric generation. The reduction of the area required for the application of nitrogen-rich digestate after the biogas production and the possibility to treat the air stripped digestate in the wastewater plant are other advantages of the combined system AS/AD [41]. The main bacteria in the anaerobic process are hydrogenotrophic Methanocorpusculum, acetoclastic Methanosaeta [88], Methanobacteria, and Methanocorpusculum [89]. Clostridia are generally the most abundant bacteria for hydrolysis and fermentation as they can tolerate high concentrations of volatile fatty acids [89,90], while the Bacilli class easily produces acetate and lactate [88]. Schröder et al. [91] used the liquid fraction of the digestate to recover phosphorous and nitrogen by struvite precipitation; then, the effluent was used for the production of ammonium sulphate by air stripping. These combined systems allowed the nearly total recovery of nutrients. By struvite precipitation, up to 90% of soluble phosphates was removed, but only less than 30% of ammonia was recovered [92], Sustainability 2020, 12, 4971 10 of 28

Air stripping has been well integrated in bio-energy production systems, as this process can be applied on digestate after biogas production (thus for biofuel and/or heat production) from organic waste or as treatment prior to the anaerobic digestion (Figure2). As a matter of fact, air stripping enhances methane yield, apart from being cost-effective for ammonia removal [32]. When air stripping is used as pre-treatment of anaerobic digestion, the main purpose is the removal of ammonia compounds that can inhibit the methanogenic activity. In this case, the dose and type of alkalis for initial pH adjustment of SW should be appropriately evaluated, in order to avoid possible inhibition of the anaerobic digestion due to cationic toxicity [25] or to an excessive nitrogen removal. The anaerobic digestion of raw SW, in addition to the difficulties mentioned above, may lead to the production of a digestate containing organic residues and ammonia compounds. Hence, a further biological (aerated) process is generally required to allow the use of the digestate as soil conditioner. To overcome this issue, air stripping can also be applied as post-treatment of anaerobic digestion to reduce the nitrogen and organic concentrations in the digestate. The stripped ammonia from both configurations (that is, from raw and digested SW) can be recovered as ammonium sulphate. Moreover, anaerobic digestion is another way to recover carbon (generally expressed in terms of chemical oxygen demand, COD) from swine wastewater, since the biodegradable carbon can be decomposed by microorganisms [85]. However, as previously mentioned, the theoretical methane 1 yield (about 380 mL gCOD− ,[86] corresponding to a complete conversion of COD), cannot be achieved due to the presence of inhibiting ammonia compounds. When air stripping is coupled with anaerobic digestion (Figure2b,c), relatively high temperatures (up to 60 ◦C) can be more easily achieved using the excess heat of the combined heat and power plant [87] or recovering heat from flue gas in conventional schemes of electric generation. The reduction of the area required for the application of nitrogen-rich digestate after the biogas production and the possibility to treat the air stripped digestate in the wastewater plant are other advantages of the combined system AS/AD [41]. The main bacteria in the anaerobic process are hydrogenotrophic Methanocorpusculum, acetoclastic Methanosaeta [88], Methanobacteria, and Methanocorpusculum [89]. Clostridia are generally the most abundant bacteria for hydrolysis and fermentation as they can tolerate high concentrations of volatile fatty acids [89,90], while the Bacilli class easily produces acetate and lactate [88]. Schröder et al. [91] used the liquid fraction of the digestate to recover phosphorous and nitrogen by struvite precipitation; then, the effluent was used for the production of ammonium sulphate by air stripping. These combined systems allowed the nearly total recovery of nutrients. By struvite precipitation, up to 90% of soluble phosphates was removed, but only less than 30% of ammonia was recovered [92], due to the equimolar stoichiometry required for the chemical precipitation of struvite [4]. The residual ammonia in soluble form can be potentially recovered by subsequent air stripping. Struvite (magnesium ammonium phosphate hexahydrate, MgNH4PO4) can be produced by chemical precipitation, which is a common method to recover phosphorous from wastewater. To facilitate the precipitation of struvite, the addition of Mg compounds (such as MgO or MgCl2) and caustic soda (to increase the pH up to 10) is needed [36]. Struvite precipitates in a 1:1:1 molar ratio following the general equation [92]:

2+ + 3 n + Mg + NH + HnPO − + 6H O MgNH 6H O + nH (6) 4 4 2 → 4 · 2 1 with n = 0, 1, or 2. Gibbs’ free energy of formation is about 3060.74 kJ mol− [93]. By the crystallisation process, over 90% of the phosphorous in wastewater can be recovered as struvite and approximately half as much nitrogen [94], because struvite contains approximately 0.5 kg of nitrogen per kg of phosphorous [35]. The scheme generally used for struvite crystallisation consists of fluidized bed or continuously stirred tank reactors [36]. Stirring facilitates the mixing of the solution and the formation of crystals [92]. 2+ 2 However, the presence of other inorganic ions, such as Ca and CO3 −, naturally present in SW, may Sustainability 2020, 12, 4971 11 of 28 complicate the precipitation of struvite by producing various mineral species [95]. Calcium ions can interact with carbonate ions to form calcite (CaCO3) according to the Equation (7) [92]:

2+ 2 Ca + CO − CaCO (7) 3 → 3 Gibbs’ free energy is equal to 1129.1 kJ mol 1. The chemical composition of struvite derives − − from its stoichiometric formula and thus its chemical characteristics are always the same. However, its production—with particular regard to purity and cristallisation operation)—is influenced by the process conditions, such as the raw magnesium compounds added (such as MgO or MgCl2), temperature as well as other inorganic ions (e.g., calcium and sodium) and caustic soda present in the alkalis used to increase pH. Microwave radiation technique is also able to improve the ammonia removal during air stripping, by decreasing the reaction time and activation energy. In a study carried out by Ata et al. [77], the ammonia removal efficiency of microwave-assisted air stripping was 25% higher compared to the conventional heating in air stripping; microwave radiation allows high mass transfer rate of ammonia. Conversely, the effect of aeration was less significant in ammonia removal, while pH, radiation time, and power value strongly affected the overall efficiency of the system [96].

3. Effect of Air Stripping on Raw SW

3.1. Characteristics of Swine Wastewater Swine wastewater consists of a mixture of urine, faeces, water spillage, residues of undigested food, antibiotic residues, and pathogenic microorganisms [25,97–99]. Total solids (TS), volatile solids (VS), and chemical oxygen demand (COD) are generally much lower than in raw manure, due to the slurry dilution with washing water [8]. The physico-chemical parameters of SW show large fluctuations (Table4), mainly due to the variability of the pigpen management practices [ 1,7]. Raw SW is generally 1 alkaline (pH of 7.0–7.5), less often slightly acidic. TS content is usually in the range 2–8 g L− , despite 1 some outliers (Table4). Ammonia nitrogen is generally high (2–10 g L − ), while phosphorous content 3 1 is usually low (0.05–0.13 PO4 −–P g L− [28]). Sustainability 2020, 12, 4971 12 of 28

Table 4. Summary of the main physico-chemical characteristics of raw swine wastewater (SW) studied in literature.

+ 3 pH TS VS NH4 -N NH3-N TAN TN CODt CODs TP PO4 −—P SW 1 1 1 1 1 1 1 1 Ref. [-] [%] [%TS] [gN L− ] [gN L− ] [gN L− ] [gN L− ] [g L− ] [g L− ] [g L− ] [g L− ] 7.4 0.5 - - - - - 3.8 0.2 50.9 5.5 18.3 3.0 - - [100] ± ± ± ± 8.0 0.1 4.2 0.5 * 2.7 0.3 *,** - 2.2 0.0 - 5.4 0.0 69.4 4.2 31.8 2.4 - - [101] ± ± ± ± ± ± ± 7.7 0.1 7.9 0.4 * 5.3 0.3 - 6.6 0.1 - 9.4 0.2 155.5 9.5 62.4 5.7 - - [102] ± ± ± ± ± ± ± 7.1 0.1 - - 0.892 0.003 ------[103] ± ± 7.2 0.1 - - 0.599 0.006 ------[103] ± ± 7.1 0.0 - - 0.346 0.014 ------[103] ± ± 7.2 18.82 * 14.69 * - - - 6.5 181.1 19.1 - - [104] 7.3 - 7.7 - 0.50–0.52 *,** 1.8–2.0 0.004–0.01 - - 10.593 13.220 6.129–7.010 - - [105] 7.8 0.4 5.52 1.47 * 3.62 0.86 *,** 3.6 1.7 - - 7.3 1.9 86.4 18.6 - - - [101] ± ± ± ± ± ± - - - - 1.207 0.111 - 2.023 0.154 22.929 .889 - 0.358 0.033 - [97] ± ± ± ± - - - - 0.603 0.008 - 0.958 0.005 8.375 0.152 - 0.216 0.004 - [9] ± ± ± ± 6.18 7.21 0.39 86.39 0.24 1.021 0.038 - - - 63.724 6.061 - - - [106] Raw ± ± ± ± 6.15 5.57 0.66 4.60 0.66 - - 1.959 0.05 - 56.109 3.794 - - - [107] ± ± ± ± 7.5 5.29 * 3.52 *,** 3.39 * - - 5.63 * 70.59 * - - - [38] 6.64 5.95 * 3.89 *,** 4.95 - - 7.6 94.2 54.2 - - [23] 6.75 0.18 2.5 0.18 63.0 2.28 2.26 * - - 3.93 * - - 0.371 0.043 * - [108] ± ± ± ± 7.60 0.15 - - 0.599 0.014 - - 0.798 0.023 2.383 0.065 - 0.043 0.00031 - [10] ± ± ± ± ± 7.3 0.5 2.04 0.29 * 1.46 0.17 *,** 2.7 0.1 - - 3.2 0.1 2.7 25.2 4.1 - - - [109] ± ± ± ± ± ± 8.6 0.1 37.1 0.2 77.6 0.2 - - 10.6 0.3 *** 29.8 1.5 **** - - - - [110] ± ± ± ± ± 6.64 5.95 3.89 ** - 4.95 - 7.6 94.2 54.2 - - [25] 7.57 0.03 - - 0.612 0.00814 - - 0.724 0.009 4.960 0.106 - 0.0614 0.00065 -[76] ± ± ± ± ± 8.11–8.27 - - 1.013–1.426 - - 1.381–2.001 5.338–7.065 - 0.0893–0.1894 0.0551–0.1397 [28] 8.02 0.11 - - - 2.74 0.017 - 3.054 0.019 23.82 2.59 10.87 0.41 0.527 0.067 - [96] ± ± ± ± ± ± 7.7 0.05 - - - - 0.730 0.051 - 3.532 0.231 - - - [111] ± ± ± 7.66 0.54 55 0.993 - - 1.180 3.625 - - - [48] 7.65 1.06 55 1.298 - - 1.636 9.579 - - - [48] 7.6 5.04 70 4.197 - - 5.564 86.569 - - - [48] 7.61 5.19 61 6.708 - - 8.349 77.886 - - - [48] Filtered - - - - - 4.0 - - - - - [84] 7.7 0.2 - - - - 0.378 0.024 - 2.756 0.184 - 0.105 0.0084 - [112] ± ± ± ± 6.6–7.3 - - 0.400–0.600 - - - 3.500–6.000 - 0.080–0.110 - [98] - 3.93 73.62 - - - - 28.000 - - - [5]

Notes: TS, VS = total and volatile solids, respectively; TAN = total ammonia nitrogen, TN = total nitrogen; CODt, CODs = total and soluble chemical oxygen demand, respectively; 1 TP = total phosphorous; * adapted data; ** on wet basis; *** g kg− of VS. Sustainability 2020, 12, 4971 13 of 28

Prior to its direct discharge into the environment or common depuration treatments, SW is generally subjected to physical processes, to reduce its polluting potential. These physical processes mainly consist of:

Homogeneous mixing [84,101,102,110,113]; • Solid–liquid separation by settlement [9,48,103,108]; • Sieving and filtration using filters with variable mesh, from 30 µm to 8 mm, to remove • straw, coarse particles, suspended solids, or large-sized materials (e.g., stones, bedding material) [5,10,25,48,76,84,98,100,111,112].

However, despite these preliminary treatments, SW still contains high concentrations of ammonia compounds and COD, very often over the limits for wastewater disposal [28]. In particular, the ammonia nitrogen is soluble and, as a consequence, its concentration in the liquid fraction may increase after filtration [38]. As mentioned above, the very high concentrations of nitrogen and COD in SW have negative impacts on the environment in the case of disposal without proper treatments.

3.2. Efficiency of Air Stripping on Raw SW A summary of literature data on AS of SW and the related conditions is reported in Table5, while the ammonia removal efficiency for the different air stripping schemes used for SW is summarised in Figure3. The e ffects of the operational parameters of air stripping on the ammonia removal efficiency are highly variable. In general, the highest rates of ammonia removal (over 90%) have been found at high temperatures (up to 80 ◦C) and/or pH (over 10.0) [38,39,114]. The control of these parameters, however, makes the treatment expensive, also considering the high reaction time adopted (up to 48 h, [23]). Sustainability 2020, 12, 4971 14 of 28

Table 5. Literature data about experimental tests of swine wastewater (SW) treatment using air stripping.

Ammonium Chemical Temperature Air Flow Rate Ammonia Removal System Notes Adjusted pH Time [h] Sulphate Ref. Addition [ C] [L L 1 min 1] Efficiency [%] ◦ air SW− − Recovery 11.5 22 7 30 * 90.3 9.5 22 55 15 * 91.0 - n.a. No [115] 10.5 22 10 30 * 90.0 9.5 22 30 30 * 90.0 1 25.0 AS 2 40.0 6 62.5 VFA production NaOH 10.5 Room 15 No [84] 24 70.0 48 77.5 96 80.0 9.5 49.3 * NaOH No [25] 10.0 70.5 * 9.5 40.4 * AD KOH 37 24 1.0 * 10.0 71.3 * No [25] AS + AD 9.5 30.5 * CaO 10.0 49.1 * None 7.7 65 - 9.5 80 4 0.05 * 69 Yes [38] Ca(OH) 2 11.5 98.8 7.2 28.0 NaOH 9.0 47.0 - 37 48 1.0 No [23] (40% w/w) 10.0 80.0 11.0 88.1 1.0 46.0 AS + gas adsorption NaOH 2.0 62.2 - 9.0 37 48 No [23] (40% w/w) 4.0 77.9 10.0 92.0 1 COD > 27 g L− None - 50 3.75 10 * 43.2–50.0 1 COD < 10 g L− None - 50 3.75 10 * > 90 Yes [48] 1 COD < 27 g L− NaOH 9.5 50 3.75 10 * > 80 0 88.2 AS + MW Power = 700 W NaOH 11.0 Not controlled 0.06 * 0.5 * 90.3 Yes [96] 1 * 91.6 97% phosphorous Struvite decomposition + AS NaOH 8.0–10.5 40–80 n.a. 800–4400 ** 80–95 * Yes [112] removal efficiency MgO > 10.0 25 3 5 94.3 Struvite crystallisation + AS - Yes [111] MgO > 10.0 45 3 5 96.7 Used for biomass Adsorption + AS n.a. 11.0 30 1 18.2 * 80.5 Yes [10] production Notes: AS = air stripping; AD = anaerobic digestion; MW = microwave radiation; VFA = very fatty acids; n.a. = not available; * = adapted data; ** = gas – liquid volume ratio. Sustainability 2020, 12, x FOR PEER REVIEW 14 of 27

Power = 700 Not 0.5 * 90.3 W controlled 1 * 91.6 97% Struvite phosphorous 800– decomposition NaOH 8.0–10.5 40–80 n.a. 80–95 * Yes [112] removal 4400 ** + AS efficiency Struvite MgO > 10.0 25 3 5 94.3 crystallisation + - Yes [111] MgO > 10.0 45 3 5 96.7 AS Used for Adsorption + biomass n.a. 11.0 30 1 18.2 * 80.5 Yes [10] AS production Notes: AS = air stripping; AD = anaerobic digestion; MW = microwave radiation; VFA = very fatty Sustainability 2020, 12, 4971 15 of 28 acids; n.a. = not available; * = adapted data; ** = gas – liquid volume ratio.

100

80

60

40

Ammonial removal efficiency (%) 20

0 AS AS + gas Adsorption + AS AS + MW Struvite Struvite adsorption decomposition + crystallisation + AS AS System

Figure 3. Summary of the ammonia removal efficiency (mean std. dev.) data of the air stripping Figure 3. Summary of the ammonia removal efficiency (mean ±± std. dev.) data of the air stripping systemssystems for for raw raw swine swine wastewater wastewater (SW)(SW) reportedreported in literature (AS (AS == airair st stripping;ripping; MW MW = microwave= microwave radiation;radiation; AD AD= =anaerobic anaerobic digestion).digestion).

Lime,Lime, caustic caustic soda,soda, and calcium calcium hydroxide hydroxide are are commonly commonly used used for increasing for increasing pH, generally pH, generally up upto to 10.0 10.0 or or even even more, more, since, since, starting starting from from arou aroundnd 9.3, 9.3, the the dissolved dissolved ammonium ammonium is isquantitatively quantitatively transformedtransformed to to ammonia ammonia gas gas [27 ].[27]. When When pH waspH was increased increased to 9.5, to no 9.5, significant no significant differences differences in ammonia in removalammonia rates removal were found rates bywere Bonmat foundí andby Bonmatí Flotats [ 38and] compared Flotats [38] to compared the test without to the pHtest control; without instead, pH highercontrol; removal instead, effi higherciencies removal (in terms efficiencies of nitrogen (in te andrms COD)of nitrogen were and detected COD) whenwere detected pH was correctedwhen pH to 11.5.was At corrected this pH, to high 11.5. ammoniaAt this pH, removal high ammonia can be removal obtained can even be obtained at lower even temperatures at lower temperatures (about 20 ◦ C), but(about operational 20 °C), but drawbacks operational are expected,drawbacks due are toexpected, the excess due of to alkalis the excess [115 of]. Onalkalis the [115]. other On hand, the atother lower pHhand, values at lower (typical pH ofvalues raw SW)(typical ammonia of raw SW) can ammonia be completely can be removed, completely provided removed, that provided air stripping that air stripping is carried out at high temperature (80 °C) [38]. The influence of the initial pH values on is carried out at high temperature (80 ◦C) [38]. The influence of the initial pH values on ammonia removalammonia effi removalciency has efficiency been also has reported been also by reported other studies: by other Zhang studies: and Zhang Jahng and [25 Jahng] obtained [25] obtained a removal a removal efficiency of about 70% at mild temperature (37 °C) and air flow rate (1 Lair L1SW−1 min1 −1) and efficiency of about 70% at mild temperature (37 ◦C) and air flow rate (1 Lair LSW− min− ) and pH pH set at 10.0 in 24 h; increasing reaction time to 48 h and pH to 11, Zhang et al. [23] showed a removal set at 10.0 in 24 h; increasing reaction time to 48 h and pH to 11, Zhang et al. [23] showed a removal efficiency of 80%. However, the highest ammonia reduction (92%) was observed at the highest air efficiency of 80%. However, the highest ammonia reduction (92%) was observed at the highest air flow rate (10 Lair LSW−1 min−1) and pH adjusted to 9.0. These results confirm the influence of the aeration flow rate (10 L L 1 min 1) and pH adjusted to 9.0. These results confirm the influence of the rate on the ammoniaair SW− removal− rate [23]. Moreover, higher pH values improve the process kinetics, aeration rate on the ammonia removal rate [23]. Moreover, higher pH values improve the process reducing the reaction time needed for a pre-set ammonia removal [48]. Increasing the pH from the kinetics,natural reducingvalue (about the 7.0) reaction to 11.0, time the rate needed of ammonia for a pre-set removal ammonia increases removal nearly 8-fold [48]. Increasing[23]. At pH over the pH from10.5, the the natural ammonia value removal (about efficiency 7.0) to 11.0,is directly the rate dependent of ammonia upon removalthe temperatures increases of nearlythe influent 8-fold air [23 ]. Atand pH liquid over 10.5,[38,115]. the At ammonia this pH level removal (over e 10.0)fficiency and room is directly temperature, dependent in a study upon by the Yang temperatures et al. [84], of the influent air and liquid [38,115]. At this pH level (over 10.0) and room temperature, in a study by Yang et al. [84], more than 90 h were required to achieve 80% of ammonia removal with a high flow rate 1 1 (15 L L− min− ). Conversely, about 65–70% of ammonia recovery was obtained at a lower pH (7.7–9.5) 1 1 and temperature of 80 ◦C, using a very low air flow rate (0.05 Lair LSW− min− ) in 4 h only [38]. The alkali type affects the process efficiency; in particular, sodium and potassium hydroxides allow higher ammonia removal rates compared to lime [25]. The choice of the alkali type is particularly important when air stripping is used as a pre-treatment for SW before AD, as the excess of light metals ions (i.e., Ca or Na due to the solubilisation of salts) can inhibit the process. Changes in pH during air stripping are expected, because the temperature drives the chemical equilibrium among the reagents leading to a pH modification; for instance, the formation and accumulation of very fatty acids (VFA) can lead to a pH reduction [38]. Moreover, pH can increase due to concentration of alkali solution caused by water evaporation [38]. Sustainability 2020, 12, 4971 16 of 28

As mentioned in Section 2.2.2., air stripping can be integrated with other processes, such as microwave radiation or chemical precipitation for recovering struvite. However, La et a. [96] found that, in the integrated system air stripping plus microwave radiation, the effect of aeration was not significant in the removal efficiency of nitrogen, while the initial pH of the SW, radiation time, and applied power noticeably influenced this efficiency. Chemical precipitation is more often coupled with air stripping for the simultaneous recovery of nitrogen and phosphorous, allowing a high removal of nutrients. Huang et al. [112] removed over 90% of total ammonia nitrogen and approximately 97% of total phosphorous by an integrated reactor. In a study of Huang et al. [111], phosphate was first recovered by chemical precipitation, then the supernatant was subjected to air stripping for ammonia recovery. The best conditions allowing a reduction in ammonia of 94% in 3 h, consisted of an air flow rate of 1 1 1 5 L L− min− , a dose of 8 g L− of MgO for pH adjustment (over 10.0), and a temperature of 25 ◦C. Cao et al. [10] used an adsorbing-stripping stage that efficiently removed nutrients (about 80% of ammonia) and heavy metals from SW, while obtaining the maximum amount of biomass to obtain various bio-products. The produced biomass was Chlorella vulgaris, an unicellular and autotrophic photosynthetic green algae, which can synthesises lipids, carbohydrates, and protein with water, nitrogen, phosphorus, sun-light, and CO2 through photosynthesis [10]. In this case, air stripping acts as a pre-treatment (Figure2), since the excessive nutrient concentration in swine wastewater inhibits microalgae growth. In this study, the treatment of SW using the combined system N-adsorption plus air stripping plus microalgae cultivation was proposed as an alternative to the common anaerobic digestion process of raw SW. Microalgae cultivation would be, indeed, a sustainable technology to recover the carbon from swine wastewater [85]. Microalgae can uptake both inorganic carbon (autotrophic metabolism) in the presence of light or organic carbon (heterotrophic metabolism) when light is absent [85]. A combined system ammonia removal plus anaerobic digestion plus microalgae growth was developed by Zhang et al. [116], with methane and biomass production from the organic matter fed to anaerobic digestion. Several cultivations of microalgae were used for the treatment of SW [85], mainly Chlorella vulgaris [117], Neochloris aquatica CL-M1 [118], Parachlorella kessleri QWY28 [119], Coelastrella sp. QY01 [120], or blooms of Chlorella vulgaris, Scenedesmus obliquus, and Pseudokirchneriella subcapitata with the fungus Ganoderma lucidum [121]. The efficiency in COD reduction was in the range of 63–88%. The removal efficiency as well as the quality of the ammonia solution recovered by air stripping are also influenced by the 1 SW characteristics. More specifically, air stripping of SW with low organic matter (<10 gCOD L− ) and without pH modification showed a removal efficiency over 90%, while higher organic matter content 1 (>27.0 gCOD L− ) reduced the nitrogen removal efficiency down to 50% or less [48]. Increasing pH to 1 9.5 significantly improved air stripping with removal efficiencies over 80% for COD below 40 gCOD L− after a treatment of about 4 h [48]. The organic compounds stripped from the SW by air stripping could affect the quality of the recovered ammonium sulphate. For this reason, an alkaline trap (pH of the solution over 12) before the acidic trap can be used to absorb the stripped volatile compounds (retention of over 60% of the stripped organics) to avoid salt contamination [48]. Although the air stripping is primarily used to remove ammonia compounds, this process is also effective to reduce the organic matter load of wastewater. In the studies reported in Table3, the COD removal efficiency varied from less than 10% to 30%. The main reasons of the reduction in organic compounds by air stripping process were the stripping of volatile compounds, such as VFA, and the aerobic biological degradation [23,25,38].

4. Effect of Air Stripping on Digested SW

4.1. Characteristics of Digested SW Although being the product of a biochemical treatment (Figure2c), anaerobically digested SW is still characterised by a high concentration of nitrogen compounds [122,123] (Table6). In more detail, almost all organic phosphorous and nitrogen in the SW are transformed into phosphate and ammonia Sustainability 2020, 12, 4971 17 of 28 through the anaerobic process [124], due to the low conversion into biomass compared to the aerobic processes. It follows that, in spite of the biogas production, the anaerobic digestion cannot effectively reduce the nutrient load [125] and especially ammonia accumulates in digesters (usual concentrations 1 1 range between 300 mg L− and 3000 mg L− or even more [126]). As a consequence, as happens for raw SW, digested SW requires further treatments before its release into the environment [10,125,127] (Figure2). Air stripping is considered as a more sustainable treatment for digested SW to remove nitrogen compared to other systems, due to (i) the waste heat produced by on-site biogas combustion that is usable to increase the temperature during air stripping, (ii) lower alkali requirement for pH adjustment, and (iii) more sustainable investment costs [38,41]. On the other hand, the digestate stripping is limited by specific constraints, such as the high content of solids, which causes clogging or scaling of the equipment and limits the application of the commonly used packed bed columns [87]. For this reason, digested SW is generally subjected to solid–liquid separation by settling [122] or filtration to remove solids [127], before being processed by air stripping.

4.2. Efficiency of Air Stripping on Digested SW A summary of literature data on air stripping of digested SW and the related process conditions is reported in Table7, while the ammonia removal e fficiency of the different air stripping schemes used for digested SW is summarised in Figure4. Before discussing the e fficiency of the studied systems, it may be worth to mention that the digestate used in the experiments was often derived from the co-digestion of SW with other substrates, such as glycerin, exhausted oil, food processing or slaughterhouse waste [41], pig manure, maize silage, sugar and pig fodder [87], pig excreta and kitchen waste [39], manure, and flushing water of a swine farm [122]. Less commonly, the analysed digested SW derives from the anaerobic digestion of SW only (e.g., [127,128]). Sustainability 2020, 12, 4971 18 of 28

Table 6. Summary of the main physico-chemical characteristics of digested swine wastewater (SW) studied in literature experiences.

+ 3 Digestate pH TS VS NH4 -N NH3-N TAN TN CODt CODs PO4 −—P 1 1 1 1 1 1 TP 1 Ref. Type [-] [%] [%TS] [gN L− ] [gN L− ] [gN L− ] [gN L− ] [g L− ] [g L− ] [g L− ] 8.2–8.5 - 0.36–0.42 *,** 2.104–2.111 0.916–0.920 - - 7.134–7.924 3.138–4.889 --[105] 8.4 3.172 * 1.717 *,** 3.68 * - - 4.73 * - 41.23 * - - [38] 8.1 0.08 4.2 0.44 58.2 0.89 3.4 * - - 4.6 * - - 1.137 0.355 * - [108] ± ± ± ± 8.17 1.48 51 3.013 - - 3.415 14.943 - - - [48] DSW 8.75 1.13 48 2.686 - - 3.353 9.790 - - - [48] 7.63 0.04 1.149 0.0724 * 0.0461 0.0012 *,** 0.298 0.0024 - - 0.460 0.0092 1.602 0.032 - 1.14.9 0.00724 - [127] ± ± ± ± ± ± ± 7.18 0.18 - - 0.706 0. 216 - - - 2.108 0. 479 - - 0.0403 0.0095 [124] ± ± ± ± 7.3–8.0 - - - > 0.160 - - 0.150–0.500 - > 0.03 - [128] 7.76 0.09 - - - - 0.874 0.112 0.968 0.123 1.595 0.361 - - - [122] ± ± ± ± Co-DSW 7.94 0.070 * 0.032 *,** 2.2 - - - 5.4 - - - [41] 7.50 - - 1.510 0.034 - 1.770 2.290 - 0.432 0.227 [39] Notes: TS = Total Solids; VS = Volatile Solids; TAN = Total Ammonia Nitrogen; TN = Total Nitrogen; COD, CODt = total and soluble COD; TP = Total Phosphorous; DSW = digested swine wastewater; Co-DSW = Co-Digested swine wastewater.

Table 7. Literature data about experimental tests of digested swine wastewater (SW) treatment using air stripping.

Temperature Air Flow Rate Ammonia Removal Ammonium Sulphate/ System Notes Chemical Addition Adjusted pH Time [h] 1 1 Ref. [◦C] [Lair LSW− min− ] Efficiency [%] Nutrient Recovery 8.5 27 50 1875 *** 10.5 93 Continuous bench plant 30 30 NaOH 10.0 n.a. 1875 *** Yes [41] spraying DSW 70 92 412 *** 55 10.0 50 2100 *** 88 12.0 15 24 0 25 CO stripping + AS + Biogas 12.0 15 24 3 72 AS 2 Ca(OH) Yes [39] purification 2 12.0 15 12 5 90 12.0 15 12 10 95 Biogas flow 47 None >7.5 65 4 5 No [87] Flue gas flow 86 10.5 2 2000 67 11.0 2 2000 69 - NaOH 30 Yes [129] 11.5 2 2000 71 12.0 2 2000 74 Sustainability 2020, 12, 4971 19 of 28

Table 7. Cont.

Temperature Air Flow Rate Ammonia Removal Ammonium Sulphate/ System Notes Chemical Addition Adjusted pH Time [h] 1 1 Ref. [◦C] [Lair LSW− min− ] Efficiency [%] Nutrient Recovery None 8.5 5 >96 Filtration before AS 9.5 80 0.05 * >96 Yes [38] Ca(OH) 4 AS + adsorption 2 11.5 >96 >8.0 50 3.75 10 * >90 COD < 10 g L 1 NoneNaOH Yes [48] − 9.5 50 3.75 10 * >90 AS + struvite - MgO 9–10 40 3 8 88 ** Yes [127] Ca(OH) >11.0 28–30 1 13 *,*** 94 Simultaneous removal of N, P 2 Chemical precipitation + AS Ca(OH) >11.0 28–30 1 39 *,*** >92 Yes [128] and COD by a WSA 2 Ca(OH)2 >11.0 28–30 1 9 *,*** >92 * CO stripping—SBR None - 0–20 1–4 0.25 * 40–90 Struvite crystallization + AS 2 Yes [124] Continuous flow—HRT = 6–15 h None - 0–30 - 0.42 * 40–90 10.0–10.5 30 n.a. 2800 *** 83 AS + MBR Continuous process NaOH No [122] 9.0–9.5 30 n.a. 2800 *** 65 1 1 1 1 Notes: * Adapted/extrapolated data; ** Ammonium recovery in phosphoric acid and struvite precipitation; *** air to liquid ratio [Lair min− :LDSW min− or m sec− : m sec− ]; AS = air stripping; WSA = water sperged aerocyclone; SBR = sequencing batch reactor; MBR = membrane bioreactor; DSW = digested swine wastewater; HRT = hydraulic retention time. Sustainability 2020, 12, 4971 20 of 28 Sustainability 2020, 12, x FOR PEER REVIEW 19 of 27

100

80

60

40

Ammonial removal efficiency Ammonial(%) removal efficiency 20

0 AS AS + adsorption AS + gas AS + struvite Chemical Struvite adsorption precipitation + AS crystallisation + AS System

Figure 4. Summary of the ammonia removal efficiency (mean std. dev.) of the air stripping systems Figure 4. Summary of the ammonia removal efficiency (mean ±± std. dev.) of the air stripping systems forfor digested digested swine swine wastewater wastewater (SW)(SW) reportedreported in literature (AS (AS == airair stripping; stripping; MBR MBR = =membranemembrane bioreactor).bioreactor).

TheThe eff effectsects of of air air stripping stripping on on digested digested SW SW are ar dieff erentdifferent from from those those observed observed in raw in SW.raw InSW. general, In a lowergeneral, reaction a lower time reaction (less time than (less 24 h) than was 24 required h) was torequired achieve to aachieve higher a ammonia higher ammonia removal removal efficiency (overefficiency 90%); moreover,(over 90%); pH moreover, seems topH influence seems to lessinflue thence overall less the treatment overall treatment efficiency, efficiency, although although higher pH valueshigher were pH evaluesffective were in improvingeffective in theimproving process the kinetics process [48 kinetics]. [48]. AnAn almost almost complete complete ammonia ammonia removalremoval was achieved by by Bonmatí Bonmat andí and Flotats Flotats [38], [38 by], by setting setting the the initialinitial pH pH at 9.5,at 9.5, but but no furtherno further increases increases were were reported reported when when the initialthe initial pH was pH setwas at set 11.5. at This11.5. showedThis that,showed at a temperature that, at a temperature of 80 ◦C, the of process80 °C, the requires process a pHrequires higher a pH than higher 9.5. Lowering than 9.5. theLowering temperature the temperature at 50 °C leads to an ammonia removal of 75% [41]. Similar results were found by Hidalgo at 50 ◦C leads to an ammonia removal of 75% [41]. Similar results were found by Hidalgo et al. [45], whoet al. reported [45], who that reported a pH over that 10.5a pH did over not 10.5 aff diectd thenot balanceaffect the between balance between molecular molecular ammonia ammonia and ionic ammoniumand ionic [ammonium32]. Conversely, [32]. Conversely, Guštin and Marinšek-LogarGuštin and Marinšek-Logar [41] reported [41] that reported pH had that the strongestpH had the eff ect onstrongest stripping, effect with on an stripping, ammonium with removal an ammonium up to 93%, removal while up the to 93%, temperature while the had temperature the least significanthad the least significant effect. In general, the pH adjustments in the chemical treatments increases the effect. In general, the pH adjustments in the chemical treatments increases the alkalinity of digested alkalinity of digested SW (over 9.0–10.0), which requires further treatments before disposal or reuse, SW (over 9.0–10.0), which requires further treatments before disposal or reuse, such as extensive such as extensive depuration (e.g., lagooning) or neutralisation. depuration (e.g., lagooning) or neutralisation. Lei et al. [39] obtained similar results (95% of removal efficiency) at room temperature, but Lei et al. [39] obtained similar results (95% of removal efficiency) at room temperature, but adopting adopting very high pH (up to 12.0) and AFR (up to 10 Lair LSW−1 min−1 for 12 h). These results indicate 1 1 verythat, high at the pH same (up topH 12.0) conditions, and AFR the (up ammonia to 10 L airremovalLSW− increasesmin− for with 12 h).the These AFR. resultsHowever, indicate a lower that, at the same pH conditions, the ammonia removal increases with the AFR. However, a lower AFR (e.g., AFR (e.g., 1 or 5 Lair LSW−1 min−1) still ensured high removal efficiencies (up to 90%), resulting in a 1 1 1 ormore 5 L airfeasibleLSW− processmin− ) [32,39]. still ensured The same high conclusi removalons effi wereciencies reported (up to by 90%), Guštin resulting and Marinšek-Logar in a more feasible process[41], who [32 ,did39]. not The observe same conclusionsany increases were in ammonia reported removal by Guštin efficiency and Marinšek-Logarraising the air-to-liquid [41], who ratio did notfrom observe 1875 up any to increases 2100. in ammonia removal efficiency raising the air-to-liquid ratio from 1875 up to 2100.Struvite precipitation is the most used physico-chemical process in combination with air strippingStruvite for precipitation the simultaneous is the mostremoval used of physico-chemicalammonia nitrogen process and phosphate in combination from digested with air SW stripping (as forfor the raw simultaneous SW, see Section removal 3.2). of Cao ammonia et al. nitrogen[127] used and MgO phosphate as chemical from additive digested for SW both (as for struvite raw SW, seeprecipitation Section 3.2 ).and Cao pH et adjustment al. [127] used for MgOair stripping as chemical of the additive residual for ammonia. both struvite The optimal precipitation process and pHconditions, adjustment allowing for air an stripping ammonia of removal the residual efficiency ammonia. of about The 90%, optimal wereprocess a temperature conditions, of 40 allowing°C, an AFR of 8 L L−1 min−1, a reaction time of 3 h, and a dose of MgO equal to 0.75 g L−1. Quan et al. 1[128] 1 an ammonia removal efficiency of about 90%, were a temperature of 40 ◦C, an AFR of 8 L L− min− , used a water-sparged aerocyclone reactor for air stripping after1 struvite precipitation. Ca(OH)2 was a reaction time of 3 h, and a dose of MgO equal to 0.75 g L− . Quan et al. [128] used a water-sparged used as precipitant for NH4+, PO43−, and organic phosphorous compounds and as pH conditioner for aerocyclone reactor for air stripping after struvite precipitation. Ca(OH)2 was used as precipitant for + 3 NH4 , PO4 −, and organic phosphorous compounds and as pH conditioner for the air stripping of Sustainability 2020, 12, 4971 21 of 28 residual ammonia; an ammonia removal over 90% was achieved in less than 3 h. Another air stripping system used in combination with a membrane bioreactor allowed a nitrogen removal from digested SW higher than 80% [122]. In order to avoid or at least reduce the need for chemicals as much as possible, a method to adjust pH may be the use of the biogas and/or the flue gas derived from biogas combustion. Before air stripping, the pH of water increases, when the amount of CO2 dissolved in the wastewater decreases. Therefore, Lei et al. [39] increased the pH of the digested effluent (from 7.4 up to 9.3) by CO2 stripping 1 1 (at the rate of 2.5 Lair LDSW− min− for one day), thus reducing the required dose of alkalis. After air stripping, the effluent was subjected to biogas injection, in order to decrease the pH of the treated digested SW for the subsequent biological treatment or the disposal into water bodies, and at the same time to purify the biogas. The pH of the effluent was reduced from 11.0 to 7.0 in less than 0.5 h (biogas 1 1 flow rate equal to 1 L L− min− ) and the methane content in the biogas was increased up to about 75%. Song et al. [124] recovered up to 85% of phosphate as struvite and removed up to 90% of ammonia nitrogen by air stripping without adding any chemicals, as the wastewater pH was increased by CO2 stripping. Moreover, Bousek et al. [87] found that the flue gas (on average 82% of nitrogen and 18% of carbon dioxide) cannot replace the air in air stripping, since the ammonia removal efficiency was lower than 50% compared to a value of 86% using air. As for raw SW, Laureni et al. [48] evaluated the quality of the ammonium sulphate solution from air stripping of digested SW. It was found that, although the total organic carbon concentrations were below the detection limit, the organic matter contamination of the solution strongly depends on the initial organic matter content and pH. It was suggested that a higher pH could also limit the contamination of the recovered product. Air stripping of digested SW 1 with an initial organic matter content below 10 gCOD L− showed over 90% of ammonia removal (in less than 4 h at an initial pH of 9.5 [48]). The COD reduction in digested SW after air stripping was in the range 20–70% according to the studies of Bonmatí and Flotats [38], Quan et al. [128], and Cao et al. [127]. In the study of Bonmatí and Flotats [38], the high removal of organic matter may be attributed to the high biomass content in the digested SW, which can adsorb suspended solids, subsequently removed by filtration.

5. Future Perspectives Air stripping of raw and digested swine wastewater can be well integrated in the concept of bio-refinery, because this system allows a more sustainable management of the piggery effluent by turning wastewater into new added-value compounds, by-products, and/or bioenergy. Moreover, the combination of air stripping with other technologies seems to be sustainable from both the environmental and economic aspects. The common combined systems include struvite precipitation and air stripping as pre- or post-treatment of the anaerobic digestion. Less commonly, air stripping was used in combination with biomass (e.g., microalgae) production. The recovery of phosphorous by chemical precipitation of struvite, and the subsequent production of ammonia salts by air stripping is a sustainable option for the recovery of both nutrients, although usually phosphorus concentration in swine wastewater is moderate. Regarding the anaerobic digestion, the application of the air stripping as pre-treatment can increase the methane yield by reducing the inhibitory ammonia compounds. When applied on the digestate (as post-treatment of the anaerobic digestion), the recovery of nitrogen makes the digestate more suitable for direct land application; however, this opportunity needs a deeper insight. A possible solution could be the recovery of both struvite (before the anaerobic digestion) and nitrogen (on the struvite-pretreated swine wastewater subjected to anaerobic digestion). However, many other solutions can be developed since air stripping offers flexible and simple opportunities of implementation. Beside the common parameters (pH, temperature, and air flow rate), the presence of organic matter of swine wastewater plays an important role on ammonia removal efficiency (for both raw and digested swine wastewater). However, the influence of the organic matter on this process has not been extensively studied. Thus, further research should evaluate the impact of the stripped organic volatile Sustainability 2020, 12, 4971 22 of 28 compounds on the quality of the ammonia salt produced by the air stripping as well as the agronomic effects of land application of the recovered by-products as fertilisers. Overall, this review has demonstrated that the air stripping process as a treatment of raw and digested swine wastewater has not been exhaustively investigated. For instance, few experiments were found on the coupled system air stripping–biomass growth and optimized air stripping–anaerobic digestion as well as on the use of other technologies (e.g., microwave radiation). The possible nutrient recovery and biomass harvest (to obtain energy or by-products) would be of great interest on a circular economy approach. Under this aspect, a special effort is required to demonstrate the overall competitiveness on the market of fertilisers of the recovered ammonia salts.

6. Conclusions This review has analysed and summarised the main studies of air stripping carried out on raw and digested swine wastewater. The related experiments were performed under a large variety of operational conditions: Natural (uncontrolled) or adjusted pH (up to 12), room temperature or 1 1 wastewater heating up 80 ◦C, air flow rate from 0.5 to 10 L L− min− , and initial ammonia content 1 from 1.0 to 7.0 g L− . The combination of these environmental conditions led to ammonia removal efficiencies from about 20% to nearly 100%. These results and the comparisons of literature experiences on raw (Section 3.2) and digested (Section 4.2) swine wastewater suggested the following indications, which could help the optimization of the air stripping treatment. The initial pH value of the wastewater before air stripping is the most influencing parameter, whose optimal value (9.5–10.0) allows an efficient and quick ammonia removal. At pH under 9.5, temperatures over 50 ◦C and/or air flow rates 1 1 up to 5 Lair LSW− min− are needed for an efficient process. An excess of alkali, although causing drawbacks (e.g., membrane fouling), noticeably reduce the costs of aeration and heating. Moreover, no pH adjustment of the air stripping treated wastewater is needed when anaerobic digestion follows air stripping, since the pH of the pre-treated wastewater is generally optimal for an efficient anaerobic digestion. Digested swine wastewater is instead less influenced by the initial pH value.

Author Contributions: Conceptualization, P.S.C. and D.A.Z.; methodology, P.S.C. and D.A.Z.; formal analysis, P.S.C. and D.A.Z.; data curation, A.F.; writing—original draft preparation, A.F.; writing—review and editing, A.F., P.S.C., and D.A.Z.; supervision, P.S.C. and D.A.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Kim, J.-H.; Chen, M.; Kishida, N.; Sudo, R. Integrated real-time control strategy for nitrogen removal in swine wastewater treatment using sequencing batch reactors. Water Res. 2004, 38, 3340–3348. [CrossRef] 2. Hill, V.R. Prospects for pathogen reductions in livestock wastewaters: A review. Crit. Rev. Environ. Sci. Technol. 2003, 33, 187–235. [CrossRef] 3. Mores, R.; Treichel, H.; Augusto, C.; Kunz, A.; Steffens, J.; Marcos, R. Remove of phosphorous and turbidity of swine wastewater using electrocoagulation under continuous flow. Sep. Purif. Technol. 2016, 171, 112–117. [CrossRef] 4. Mehta, C.M.; Khunjar, W.O.; Nguyen, V.; Tait, S.; Batstone, D.J. Technologies to recover nutrients from waste streams: A critical review. Crit. Rev. Environ. Sci. Technol. 2015, 45, 385–427. [CrossRef] 5. Safavi, S.M.; Unnthorsson, R. Enhanced methane production from pig slurry with pulsed electric field pre-treatment. Environ. Technol. 2017, 3330, 1–11. [CrossRef] 6. Motteran, F.; Pereira, E.L.; Campos, C.M.M. The behaviour of an anaerobic baffled reactor (ABR) as the first stage in the biological treatment of hog farming effluent. Braz. J. Chem. Eng. 2013, 30, 299–310. [CrossRef] 7. Cheng, N.; Lo, K.V.; Yip, K.H. Swine wastewater treatment in a two stage sequencing batch reactor using real-time control. J. Environ. Sci. Heal.—Part B Pestic. Food Contam. Agric. Wastes 2000, 35, 379–398. [CrossRef] Sustainability 2020, 12, 4971 23 of 28

8. Cai, J.; Mo, X.; Cheng, G.; Du, D. Pretreatment of piggery wastewater by a stable constructed microbial consortium for improving the methane production. Water Sci. Technol. 2015, 71, 769–776. [CrossRef] 9. Yang, D.; Deng, L.; Zheng, D.; Wang, L.; Liu, Y. Separation of swine wastewater into different concentration fractions and its contribution to combined anaerobic-aerobic process. J. Environ. Manag. 2016, 168, 87–93. [CrossRef] 10. Cao, L.; Zhou, T.; Li, Z.; Wang, J.; Tang, J.; Ruan, R.; Liu, Y. Effect of combining adsorption-stripping treatment with acidification on the growth of Chlorella vulgaris and nutrient removal from swine wastewater. Bioresour. Technol. 2018, 263, 10–16. [CrossRef] 11. De Haas, D.W.; Wentzel, M.C.; Ekama, G.A. The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate removal. Part 1: Literature review. Water SA 2001, 27, 151–165. 12. Liu, Y.; Shi, H.; Li, W.; Hou, Y.; He, M. Inhibition of chemical dose in biological phosphorus and nitrogen removal in simultaneous chemical precipitation for phosphorus removal. Bioresour. Technol. 2011, 102, 4008–4012. [CrossRef][PubMed] 13. Cox, A.E.; Camberato, J.J.; Smith, B.R. Phosphate availability and inorganic transformation in an alum sludge-affected soil. J. Environ. Qual. 1997, 26, 1393–1398. [CrossRef] 14. Loughrin, J.H.; Quintanar, A.I.; Cook, K.L.; Lovanh, N.C.; Lane, B. Seasonal variation in heat fluxes, predicted emissions of malodorants, and wastewater quality of an anaerobic swine waste lagoon. Water Air Soil Pollut. 2012, 223, 3611–3618. [CrossRef] 15. Trias, M.; Hu, Z.; Mortula, M.M.; Gordon, R.J.; Gagnon, G.A. Impact of seasonal variation on treatment of swine wastewater. Environ. Technol. 2004, 25, 775–781. [CrossRef][PubMed] 16. Craggs, R.A.; Park, J.A.; Heubeck, S.A. Methane emissions from anaerobic ponds on a piggery and a dairy farm in New Zealand. Aust. J. Exp. Agric. 2008, 48, 142–146. [CrossRef] 17. Zema, D.A.; Andiloro, S.; Bombino, G.; Tamburino, V.; Sidari, R.; Caridi, A. Depuration in aerated ponds of citrus processing wastewater with a high concentration of essential oils. Environ. Technol. 2012, 33, 1255–1260. [CrossRef] 18. Andiloro, S.; Bombino, G.; Tamburino, V.; Zema, D.A.; Zimbone, S.M. Aerated lagooning of agro-industrial wastewater: Depuration performance and energy requirements. J. Agric. Eng. 2013, 44, 827–832. [CrossRef] 19. Zema, D.A.; Andiloro, S.; Bombino, G.; Caridi, A.; Sidari, R.; Tamburino, V. Comparing different schemes of agricultural wastewater lagooning: Depuration performance and microbiological characteristics. Water Air Soil Pollut. 2016, 439, 227–439. [CrossRef] 20. Calabrò, P.S.; Fòlino, A.; Tamburino, V.; Zappia, G.; Zema, D.A. Increasing the tolerance to polyphenols of the anaerobic digestion of olive wastewater through microbial adaptation. Biosyst. Eng. 2018, 172, 19–28. [CrossRef] 21. Zema, D.A.; Fòlino, A.; Zappia, G.; Calabrò, P.S.; Tamburino, V.; Zimbone, S.M. Anaerobic digestion of orange peel in a semi-continuous pilot plant: An environmentally sound way of citrus waste management in agro-ecosystems. Sci. Total Environ. 2018, 630.[CrossRef] 22. Zema, D.A.; Calabro, P.S.; Folino, A.; Tamburino, V.; Zappia, G.; Zimbone, S.M. Wastewater management in citrus processing industries: An overview of advantages and limits. Water 2019, 11, 2481. [CrossRef] 23. Zhang, L.; Lee, J.W.; Jahng, D. Ammonia stripping for enhanced biomethanization of piggery wastewater. J. Hazard. Mater. 2012, 199–200, 36–42. [CrossRef][PubMed] 24. Hansen, K.H.; Angelidaki, I.; Ahring, B.K. Anaerobic digestion of swine manure: Inhibition by ammonia. Water Res. 1998, 32, 5–12. [CrossRef] 25. Zhang, L.; Jahng, D. Enhanced anaerobic digestion of piggery wastewater by ammonia stripping: Effects of alkali types. J. Hazard. Mater. 2010, 182, 536–543. [CrossRef][PubMed] 26. Browne, J.D.; Allen, E.; Murphy, J.D. Evaluation of the biomethane potential from multiple waste streams for a proposed community scale anaerobic digester. Environ. Technol. (United Kingdom) 2013, 34, 2027–2038. [CrossRef][PubMed] 27. Sengupta, S.; Nawaz, T.; Beaudry, J. Nitrogen and Phosphorus Recovery from Wastewater. Curr. Pollut. Rep. 2015, 1, 155–166. [CrossRef] 28. Zhang, D.; Chen, Y.; Jilani, G.; Wu, W.; Liu, W.; Han, Z. Optimization of struvite crystallization protocol for pretreating the swine wastewater and its impact on subsequent anaerobic biodegradation of pollutants. Bioresour. Technol. 2012, 116, 386–395. [CrossRef] Sustainability 2020, 12, 4971 24 of 28

29. Perera, M.K.; Englehardt, J.D.; Dvorak, A.C. Technologies for recovering nutrients from wastewater: A critical review. Environ. Eng. Sci. 2019, 36, 511–529. [CrossRef] 30. Sagberg, P.; Berg, K.G. Cost optimisation of nitrogen removal in a compact nitrogen and phosphorus WWTP. Water Sci. Technol. 2000, 41, 147–154. [CrossRef] 31. Karri, R.R.; Sahu, J.N.; Chimmiri, V. Critical review of abatement of ammonia from wastewater. J. Mol. Liq. 2018, 261, 21–31. [CrossRef] 32. Kinidi, L.; Tan, I.A.W.; Binti, N.; Wahab, A.; Fikri, K.; Tamrin, B.; Hipolito, C.N.; Salleh, S.F. Recent development in ammonia stripping process for industrial wastewater treatment. Int. J. Chem. Eng. 2018, 2018, 1–14. [CrossRef] 33. Maurer, M.; Pronk, W.; Larsen, T.A. Treatment processes for source-separated urine. Water Res. 2006, 40, 3151–3166. [CrossRef][PubMed] 34. Zarebska, A.; Nieto, D.R.; Christensen, K.V.; Søtoft, L.F. Ammonium fertilizers production from manure: A critical review. Environ. Sci. Technol. 2015, 45, 1469–1521. [CrossRef] 35. Macura, B.; Johannesdottir, S.L.; Piniewski, M.; Haddaway, N.R.; Kvarnström, E. Effectiveness of ecotechnologies for recovery of nitrogen and phosphorus from anaerobic digestate and effectiveness of the recovery products as fertilisers: A systematic review protocol. Environ. Evid. 2019, 8, 1–9. [CrossRef] 36. Vaneeckhaute, C.; Lebuf, V.; Michels, E.; Belia, E.; Vanrolleghem, P.A.; Tack, F.M.G.; Meers, E. Nutrient recovery from digestate: Systematic technology review and product classification. Waste Biomass Valorization 2017, 8, 21–40. [CrossRef] 37. Ozturk, I.; Altinbas, M.; Koyuncu, I.; Arikan, O.; Gomec-Yangin, C. Advanced physico-chemical treatment experiences on young municipal landfill leachates. Waste Manag. 2003, 23, 441–446. [CrossRef] 38. Bonmatí, A.; Flotats, X. Air stripping of ammonia from pig slurry: Characterisation and feasibility as a pre- or post-treatment to mesophilic anaerobic digestion. Waste Manag. 2003, 23, 261–272. [CrossRef] 39. Lei, X.; Sugiura, N.; Feng, C.; Maekawa, T. Pretreatment of anaerobic digestion effluent with ammonia stripping and biogas purification. J. Hazard. Mater. 2007, 145, 391–397. [CrossRef][PubMed] 40. Wastewater Engineering, Treatment and Resourse Recovery, 4th ed.; McGraw-Hill Education (Ed.) Metcalf & Eddy Inc.: New York, NY, USA, 2003. 41. Guštin, S.; Marinšek-Logar, R. Effect of pH, temperature and air flow rate on the continuous ammonia stripping of the anaerobic digestion effluent. Process Saf. Environ. Prot. 2011, 89, 61–66. [CrossRef] 42. El-Gohary, F.A.; Kamel, G. Characterization and biological treatment of pre-treated landfill leachate. Ecol. Eng. 2016, 94, 268–274. [CrossRef] 43. Quan, X.; Wang, F.; Zhao, Q.; Zhao, T.; Xiang, J. Air stripping of ammonia in a water-sparged aerocyclone reactor. J. Hazard. Mater. 2009, 170, 983–988. [CrossRef] 44. Cussler, E.L. Diffusion Mass Transfer in Fluid System, 3rd ed.; Cambridge University Press, Ed.; Cambridge University Press: New York, NY, USA, 2007; ISBN 9780521871211. 45. Hidalgo, D.; Corona, F.; Martín-Marroquín, J.M.; del Álamo, J.; Aguado, A. Resource recovery from anaerobic digestate: Struvite crystallisation versus ammonia stripping. Desalin. Water Treat. 2016, 57, 2626–2632. [CrossRef] 46. Markou, G.; Agriomallou, M.; Georgakakis, D. Forced ammonia stripping from livestock wastewater: The influence of some physico-chemical parameters of the wastewater. Water Sci. Technol. 2017, 75, 686–692. [CrossRef][PubMed] 47. Saracco, G.; Genon, G. High temperature ammonia stripping and recovery from process liquid wastes. J. Hazard. Mater. 1994, 37, 191–206. [CrossRef] 48. Laureni, M.; Palatsi, J.; Llovera, M.; Bonmatí, A. Influence of pig slurry characteristics on ammonia stripping efficiencies and quality of the recovered ammonium-sulfate solution. J. Chem. Technol. Biotechnol. 2013, 88, 1654–1662. [CrossRef] 49. Desloover, J.; Abate Woldeyohannis, A.; Verstraete, W.; Boon, N.; Rabaey, K. Electrochemical resource recovery from digestate to prevent ammonia toxicity during anaerobic digestion. Environ. Sci. Technol. 2012, 46, 12209–12216. [CrossRef][PubMed] 50. Luther, A.K.; Desloover, J.; Fennell, D.E.; Rabaey, K. Electrochemically driven extraction and recovery of ammonia from human urine. Water Res. 2015, 87, 367–377. [CrossRef] Sustainability 2020, 12, 4971 25 of 28

51. Sigurnjak, I.; Brienza, C.; Snauwaert, E.; De Dobbelaere, A.; De Mey, J.; Vaneeckhaute, C.; Michels, E.; Schoumans, O.; Adani, F.; Meers, E. Production and performance of bio-based mineral fertilizers from agricultural waste using ammonia (stripping-)scrubbing technology. Waste Manag. 2019, 89, 265–274. [CrossRef] 52. Speight, J.G. Industrial inorganic chemistry. In Environmental Inorganic Chemistry for Engineers; Elsevier: Amsterdam, The Netherlands, 2017; pp. 111–169. 53. Vaneeckhaute, C.; Meers, E.; Michels, E.; Buysse, J.; Tack, F.M.G. Ecological and economic benefits of the application of bio-based mineral fertilizers in modern agriculture. Biomass Bioenergy 2013, 49, 239–248. [CrossRef] 54. Vaneeckhaute, C. Nutrient Recovery from Bio-Digestion Waste: From Field Experimentation to Model-Based Optimization. Ph.D. Thesis, Ghent University, Ghent, Belgium, 2015. 55. 55. Ammonium Sulfate Prices | Historical & Current | Intratec.us. Available online: https://www.intratec.us/ chemical-markets/ammonium-sulfate-price (accessed on 30 May 2020). 56. SE Asia Ammonium Sulphate | Fertilizerworks.com. Available online: http://fertilizerworks.com/reports/se- asia-am-sulphate (accessed on 31 May 2020). 57. GEA Messo PT Ammonium Sulfate Fertilizer Industry. Available online: https://www.gea.com/en/binaries/ ammonium-sulfate-crystallization-fertilizer-gea_tcm11-34855.pdf (accessed on 17 June 2020). 58. Kandil, A.-H.T.; Cheira, M.F.; Gado, H.S.; Soliman, M.H.; Akl, H.M. Ammonium sulfate preparation from phosphogypsum waste. J. Radiat. Res. Appl. Sci. 2017, 10, 24–33. [CrossRef] 59. Rauls, M.; Bartosch, K.; Kind, M.; Kuch, S.; Lacmann, R.; Mersmann, A. Influence of impurities on crystallization kinetics—A case study on ammonium sulfate. J. Cryst. Growth 2000, 213, 116–128. [CrossRef] 60. Maurer, M.; Schwegler, P.; Larsen, T.A. Nutrients in urine: Energetic aspects of removal and recovery. Water Sci. Technol. 2003, 48, 37–46. [CrossRef][PubMed] 61. European Union Council Directive of 12 December 1991 Concerning the Protection of Waters Against Pollution Caused by Nitrates from Agricultural Sources (91/676/EEC). Available online: https://eur-lex. europa.eu/legal-content/EN/TXT/?uri=CELEX:01991L0676-20081211 (accessed on 2 June 2020). 62. USDA-NRCS National Conservation Practice Standards | NRCS. Available online: https://www.nrcs.usda. gov/wps/portal/nrcs/main/national/technical/cp/ncps/ (accessed on 2 June 2020). 63. USDA-NRCS Agricultural Waste Management Field Handbook | NRCS. Available online: https://www.nrcs. usda.gov/wps/portal/nrcs/detailfull/national/water/?&cid=stelprdb1045935 (accessed on 2 June 2020). 64. Congreso Nacional de Bolivia. Law No. 1333—Environmental Law. 1992. Available online: https: //digitalrepository.unm.edu/la_energy_policies/271 (accessed on 18 June 2020). 65. Hernández-Padilla, F.; Margni, M.; Noyola, A.; Guereca-Hernandez, L.; Bulle, C. Assessing wastewater treatment in Latin America and the Caribbean: Enhancing life cycle assessment interpretation by regionalization and impact assessment sensibility. J. Clean. Prod. 2017, 142, 2140–2153. [CrossRef] 66. Wang, Q.; Zhang, T.; He, X.; Jiang, R. Assessment of phosphorus recovery from swine wastewater in Beijing, China. Sustainability 2017, 9, 1845. [CrossRef] 67. Ministry of Ecology and Environment. The People’s Republic of China Discharge Standard of Water Pollutants for Ammonia Industry (GB 13458-2013). Available online: http://english.mee.gov.cn/Resources/ standards/water_environment/Discharge_standard/201307/t20130705_254903.shtml (accessed on 2 June 2020). 68. Li, W.W.; Sheng, G.P.; Zeng, R.J.; Liu, X.W.; Yu, H.Q. China’s wastewater discharge standards in urbanization: Evolution, challenges and implications: Evolution, challenges and implications. Environ. Sci. Pollut. Res. 2012, 19, 1422–1431. [CrossRef][PubMed] 69. Szymanska, M.; Sosulski, T.; Szara, E.; Was, A.; Sulewski, P.; Van Pruissen, G.W.P.; Cornelissen, R.L. Ammonium sulphate from a bio-refinery system as a fertilizer-agronomic and economic effectiveness on the farm scale. Energies 2019, 12, 4721. [CrossRef] 70. Fuchs, W.; Wang, X.; Gabauer, W.; Ortner, M.; Li, Z. Tackling ammonia inhibition for efficient biogas production from chicken manure: Status and technical trends in Europe and China. Renew. Sustain. Energy Rev. 2018, 97, 186–199. [CrossRef] 71. Branch Environmentl Corp. Ammonia Stripper | Branch Environmental Corp. Available online: https: //www.branchenv.com/ammonia-stripper/ (accessed on 2 June 2020). Sustainability 2020, 12, 4971 26 of 28

72. Colsen N-Recovery | Colsen. Available online: https://www.colsen.nl/en/services/n-recovery (accessed on 2 June 2020). 73. GNS Digestate Treatment System GNS. Available online: https://www.gns-halle.de/english.html (accessed on 2 June 2020). 74. CMI Europe Environnement Treatment of Odorous and Chemical Air Pollution of Industrial and Collective Plants. Available online: http://www.europe-environnement.com/en/technologies/ (accessed on 2 June 2020). 75. Chen, L.; Cong, R.G.; Shu, B.; Mi, Z.F. A sustainable biogas model in China: The case study of Beijing Deqingyuan biogas project. Renew. Sustain. Energy Rev. 2017, 78, 773–779. [CrossRef] 76. Cao, L.; Wang, J.; Zhou, T.; Li, Z.; Xiang, S.; Xu, F.; Ruan, R.; Liu, Y. Evaluation of ammonia recovery from swine wastewater via a innovative spraying technology. Bioresour. Technol. 2019, 272, 235–240. [CrossRef] 77. Ata, O.N.; Aygun, K.; Okur, H.; Kanca, A. Determination of ammonia removal from aqueous solution and volumetric mass transfer coefficient by microwave-assisted air stripping. Int. J. Environ. Sci. Technol. 2016, 13, 2459–2466. [CrossRef] 78. Ippersiel, D.; Mondor, M.; Lamarche, F.; Tremblay, F.; Dubreuil, J.; Masse, L. Nitrogen potential recovery and concentration of ammonia from swine manure using electrodialysis coupled with air stripping. J. Environ. Manag. 2012, 95, S165–S169. [CrossRef][PubMed] 79. Tao, W.; Ukwuani, A.T.; Agyeman, F. Recovery of ammonia in anaerobic digestate using vacuum thermal stripping—Acid absorption process: Scale-up considerations. Water Sci. Technol. 2018, 78, 878–885. [CrossRef] [PubMed] 80. Ukwuani, A.T.; Tao, W. Developing a vacuum thermal stripping—Acid absorption process for ammonia recovery from anaerobic digester effluent. Water Res. 2016, 106, 108–115. [CrossRef][PubMed] 81. Degermenci,ˆ N.; Ata, O.N.; Yildiz, E. Ammonia removal by air stripping in a semi-batch jet loop reactor. J. Ind. Eng. Chem. 2012, 18, 399–404. [CrossRef] 82. Yuan, M.H.; Chen, Y.H.; Tsai, J.Y.; Chang, C.Y. Removal of ammonia from wastewater by air stripping process in laboratory and pilot scales using a rotating packed bed at ambient temperature. J. Taiwan Inst. Chem. Eng. 2016, 60, 488–495. [CrossRef] 83. Alitalo, A.; Kyrö, A.; Aura, E. Ammonia stripping of biologically treated liquid manure. J. Environ. Qual. 2012, 41, 273–280. [CrossRef] 84. Yang, K.; Oh, C.; Hwang, S. Optimizing volatile fatty acid production in partial acidogenesis of swine wastewater. Water Sci. Technol. 2004, 50, 169–176. [CrossRef] 85. Cheng, H.H.; Narindri, B.; Chu, H.; Whang, L.M. Recent advancement on biological technologies and strategies for resource recovery from swine wastewater. Bioresour. Technol. 2020, 303, 122861. [CrossRef] 86. Heidrich, E.S.; Curtis, T.P.;Dolfing, J. Determination of the internal chemical energy of wastewater. Environ. Sci. Technol. 2011, 45, 827–832. [CrossRef] 87. Bousek, J.; Scroccaro, D.; Sima, J.; Weissenbacher, N.; Fuchs, W. Influence of the gas composition on the efficiency of ammonia stripping of biogas digestate. Bioresour. Technol. 2016, 203, 259–266. [CrossRef] [PubMed] 88. Jiang, M.; Westerholm, M.; Qiao, W.; Wandera, S.M.; Dong, R. High rate anaerobic digestion of swine wastewater in an anaerobic membrane bioreactor. Energy 2020, 193, 116783. [CrossRef] 89. Wang, J.; Westerholm, M.; Qiao, W.; Mahdy, A.; Wandera, S.M.; Yin, D.; Bi, S.; Fan, R.; Dong, R. Enhancing anaerobic digestion of dairy and swine wastewater by adding trace elements: Evaluation in batch and continuous experiments. Water Sci. Technol. 2019, 80, 1662–1672. [CrossRef][PubMed] 90. Jiang, M.; Qiao, W.; Ren, Z.; Mahdy, A.; Wandera, S.M.; Li, Y.; Dong, R. Influence of operation conditions on methane production from swine wastewater treated by a self-agitation anaerobic reactor. Int. Biodeterior. Biodegrad. 2019, 143, 104710. [CrossRef] 91. Schröder, J.J.; Uenk, D.; Hilhorst, G.J. Long-term nitrogen fertilizer replacement value of cattle manures applied to cut grassland. Plant Soil 2007, 299, 83–99. [CrossRef] 92. Le Corre, K.; Valsami-Jones, E.; Hobbs, P.; S, P. Phosphorous recovery from waste water by struvite crysatllisation: A review. Crit. Rev. Environ. Sci. Technol. 2009, 39, 433–477. [CrossRef] 93. Iglesia, A. La Estimating the thermodynamic properties of phosphate minerals at high and low temperature from the sum of constituent units. Estud. Geol. 2009, 65, 109–119. [CrossRef] Sustainability 2020, 12, 4971 27 of 28

94. Rahman, M.M.; Salleh, M.A.M.; Rashid, U.; Ahsan, A.; Hossain, M.M.; Ra, C.S. Production of slow release crystal fertilizer from wastewaters through struvite crystallization—A review. Arab. J. Chem. 2014, 7, 139–155. [CrossRef] 95. Ye, Z.; Chen, S.; Lu, M.; Shi, J.; Lin, L.; Wang, S. Recovering phosphorus as struvite from the digested swine wastewater with bittern as a magnesium source. Water Sci. Technol. 2011, 64, 334–341. [CrossRef] 96. La, J.; Kim, T.; Jang, J.K.; Chang, I.S. Ammonia nitrogenrRemoval and recovery from swine wastewater by microwave radiation. Environ. Eng. Res. 2014, 19, 381–385. [CrossRef] 97. Viancelli, A.; Kunz, A.; Steinmetz, R.L.R.; Kich, J.D.; Souza, C.K.; Canal, C.W.; Coldebella, A.; Esteves, P.A.; Barardi, C.R.M. Performance of two swine manure treatment systems on chemical composition and on the reduction of pathogens. Chemosphere 2013, 90, 1539–1544. [CrossRef][PubMed] 98. Wu, D.; Zheng, S.; Ding, A.; Sun, G.; Yang, M. Performance of a zero valent iron-based anaerobic system in swine wastewater treatment. J. Hazard. Mater. 2015, 286, 1–6. [CrossRef][PubMed] 99. Cheng, D.L.; Ngo, H.H.; Guo, W.S.; Liu, Y.W.; Zhou, J.L.; Chang, S.W.; Nguyen, D.D.; Bui, X.T.; Zhang, X.B. Bioprocessing for elimination antibiotics and hormones from swine wastewater. Sci. Total Environ. 2017, 621, 1664–1682. [CrossRef] 100. Girault, R.; Rousseau, P.; Steyer, J.P.; Bernet, N.; Béline, F. Combination of batch experiments with continuous reactor data for ADM1 calibration: Application to anaerobic digestion of pig slurry. Water Sci. Technol. 2011, 63, 2575–2582. [CrossRef][PubMed] 101. Kim, W.; Shin, S.G.; Cho, K.; Lee, C.; Hwang, S. Performance of methanogenic reactors in temperature phased two-stage anaerobic digestion of swine wastewater. J. Biosci. Bioeng. 2012, 114, 635–639. [CrossRef] [PubMed] 102. Kim, W.; Cho, K.; Lee, S.; Hwang, S. Comparison of methanogenic community structure and anaerobic process performance treating swine wastewater between pilot and optimized lab scale bioreactors. Bioresour. Technol. 2013, 145, 48–56. [CrossRef] 103. Zhai, W.; Qin, T.; Guo, T.; Khan, M.I.; Tang, X.; Xu, J. Arsenic transformation in swine wastewater with low-arsenic content during anaerobic digestion. Water (Switzerland) 2017, 9, 826. [CrossRef] 104. Auphimai, C.; Rukruam, W.; Chaiprasert, P. Efficacies of various inoculum sources on methane production from agro-industrial wastewaters. Energy Procedia 2014, 52, 167–172. [CrossRef] 105. Ho, L.; Ho, G. Mitigating ammonia inhibition of thermophilic anaerobic treatment of digested piggery wastewater: Use of pH reduction, zeolite, biomass and humic acid. Water Res. 2012, 46, 4339–4350. [CrossRef] 106. Córdoba, V.; Fernández, M.; Santalla, E. The effect of different inoculums on anaerobic digestion of swine wastewater. J. Environ. Chem. Eng. 2016, 4, 115–122. [CrossRef] 107. Córdoba, V.; Fernández, M.; Santalla, E. The effect of substrate/inoculum ratio on the kinetics of methane production in swine wastewater anaerobic digestion. Environ. Sci. Pollut. Res. 2018, 25, 21308–21317. [CrossRef][PubMed] 108. Cattaneo, M.; Finzi, A.; Guido, V.; Riva, E.; Provolo, G. Effect of ammonia stripping and use of additives on separation of solids, phosphorus, copper and zinc from liquid fractions of animal slurries. Sci. Total Environ. 2019, 672, 30–39. [CrossRef] 109. Belmonte, M.; Hsieh, C.-F.; Figueroa, C.; Campos, J.L.; Vidal, G. Effect of free ammonia nitrogen on the methanogenic activity of swine wastewater. Electron. J. Biotechnol. 2011, 14.[CrossRef] 110. Huang, W.; Zhao, Z.; Yuan, T.; Lei, Z.; Cai, W.; Li, H.; Zhang, Z. Effective ammonia recovery from swine excreta through dry anaerobic digestion followed by ammonia stripping at high total solids content. Biomass Bioenergy 2016, 90, 139–147. [CrossRef] 111. Huang, H.; Guo, G.; Zhang, P.; Zhang, D.; Liu, J.; Tang, S. Feasibility of physicochemical recovery of nutrients from swine wastewater: Evaluation of three kinds of magnesium sources. J. Taiwan Inst. Chem. Eng. 2017, 70, 209–218. [CrossRef] 112. Huang, H.; Xiao, D.; Liu, J.; Hou, L.; Ding, L. Recovery and removal of nutrients from swine wastewater by using a novel integrated reactor for struvite decomposition and recycling. Sci. Rep. 2015, 5, 1–13. [CrossRef] 113. Deng, L.; Yang, H.; Liu, G.; Zheng, D.; Chen, Z.; Liu, Y.; Pu, X.; Song, L.; Wang, Z.; Lei, Y. Kinetics of temperature effects and its significance to the heating strategy for anaerobic digestion of swine wastewater. Appl. Energy 2014, 134, 349–355. [CrossRef] Sustainability 2020, 12, 4971 28 of 28

114. Van der Heyden, C.; Demeyer, P.; Volcke, E.I.P. Mitigating emissions from pig and poultry housingfacilities through air scrubbers and biofilters: State-of-the-art andperspectives. Biosyst. Eng. 2015, 134, 74–93. [CrossRef] 115. Liao, P.H.; Chen, A.; Lo, K.V. Removal of nitrogen from swine manure wastewaters by ammonia stripping. Bioresour. Technol. 1995, 54, 17–20. [CrossRef] 116. Zhang, R.; Anderson, E.; Addy, M.; Deng, X.; Kabir, F.; Lu, Q.; Ma, Y.; Cheng, Y.; Liu, Y.; Chen, P.; et al. An innovative intermittent-vacuum assisted thermophilic anaerobic digestion process for effective animal manure utilization and treatment. Bioresour. Technol. 2017, 244, 1073–1080. [CrossRef] 117. Wang, Y.; Guo, W.; Yen, H.W.; Ho, S.H.; Lo, Y.C.; Cheng, C.L.; Ren, N.; Chang, J.S. Cultivation of Chlorella vulgaris JSC-6 with swine wastewater for simultaneous nutrient/COD removal and carbohydrate production. Bioresour. Technol. 2015, 198, 619–625. [CrossRef][PubMed] 118. Wang, Y.; Ho, S.H.; Cheng, C.L.; Nagarajan, D.; Guo, W.Q.; Lin, C.; Li, S.; Ren, N.; Chang, J.S. Nutrients and COD removal of swine wastewater with an isolated microalgal strain Neochloris aquatica CL-M1 accumulating high carbohydrate content used for biobutanol production. Bioresour. Technol. 2017, 242, 7–14. [CrossRef][PubMed] 119. Qu, W.; Zhang, C.; Zhang, Y.; Ho, S.H. Optimizing real swine wastewater treatment with maximum carbohydrate production by a newly isolated indigenous microalga Parachlorella kessleri QWY28. Bioresour. Technol. 2019, 289, 121702. [CrossRef][PubMed] 120. Luo, L.; He, H.; Yang, C.; Wen, S.; Zeng, G.; Wu, M.; Zhou, Z.; Lou, W. Nutrient removal and lipid production by Coelastrella sp. in anaerobically and aerobically treated swine wastewater. Bioresour. Technol. 2016, 216, 135–141. [CrossRef] 121. Guo, G.; Cao, W.; Sun, S.; Zhao, Y.; Hu, C. Nutrient removal and biogas upgrading by integrating fungal–microalgal cultivation with anaerobically digested swine wastewater treatment. J. Appl. Phycol. 2017, 29, 2857–2866. [CrossRef] 122. Sui, Q.; Zhu, Z.; Dong, H.; Wang, Y. Nitrogen removal from anaerobically digested swine wastewater through combined ammonia stripping and membrane bioreactor processes. Trans. ASABE 2015, 58, 1289–1298. 123. Zhang, M.; Lawlor, P.G.; Hu, Z.; Zhan, X. Nutrient removal from separated pig manure digestate liquid using hybrid biofilters. Environ. Technol. (United Kingdom) 2013, 34, 645–651. [CrossRef] 124. Song, Y.; Qiu, G.; Yuan, P.; Cui, X.; Peng, J.; Zeng, P. Nutrients removal and recovery from anaerobically digested swine wastewater by struvite crystallization without chemical additions. J. Hazard. Mater. 2011, 190, 140–149. [CrossRef] 125. Lin, H.; Lin, Y.; Wang, D.; Pang, Y.; Zhang, F.; Tan, S. Ammonium removal from digested effluent of swine wastewater by using solid residue from magnesium-hydroxide flue gas desulfurization process. J. Ind. Eng. Chem. 2018, 58, 148–154. [CrossRef] 126. Ashrafizadeh, S.N.; Khorasani, Z. Ammonia removal from aqueous solutions using hollow-fiber membrane contactors. Chem. Eng. J. 2010, 162, 242–249. [CrossRef] 127. Cao, L.; Wang, J.; Xiang, S.; Huang, Z.; Ruan, R.; Liu, Y. Nutrient removal from digested swine wastewater by combining ammonia stripping with struvite precipitation. Environ. Sci. Pollut. Res. 2019, 26, 6725–6734. [CrossRef][PubMed] 128. Quan, X.; Ye, C.; Xiong, Y.; Xiang, J.; Wang, F. Simultaneous removal of ammonia, P and COD from anaerobically digested piggery wastewater using an integrated process of chemical precipitation and air stripping. J. Hazard. Mater. 2010, 178, 326–332. [CrossRef][PubMed] 129. Zou, M.; Dong, H.; Zhu, Z.; Zhan, Y. Optimization of ammonia stripping of piggery biogas slurry by response surface methodology. Int. J. Environ. Res. Public Health 2019, 16, 3819. [CrossRef][PubMed]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).